In my opinion yes you call or text the other parent. Regardless if she helps with the $ of the repair or maintenance. We literally just had the opposite happen. SK also fender bender kid nor BM notified DH. Feels kinda $hitty not knowing of your kid is going to be gun shy of driving again, needs encouragement or just a glad you are okay kid pay better attention next time from the other parent. 
sorry your DH isn't being as sensitive to you though, that does suck. As stepparents - we get cast aside too easily and quickly.  

SS didn't react quickly enough - meaning he was following too closely.  
 

Yeah, it's weird that he called BM right away. I'm not sure why he talks to her at all, honestly.  

If it was just a fender bender, nothing serious and SS was not hurt in any way, I don't think your husband needed to make it a point to call BM.

Had this happened when my SD was 16 (haha, she didn't drive until she was 21), DH would have just told her to call her mom, herself. Of course, BM over here, would have been trying to sue somebody, so maybe not.

We had 5 kids, each year, one of them turned 16, driving age here.  Without exception, every one had a minor accident and/or a ticket in the first year.  So, I guess I'm kind of blase and just expect this.

I dont see a problem with him calling BM.  If I were she, as useless and aggravating as she sounds, I'd like to know and the kid might not think to call her.  But she definitely sounds like a useless individual and I don't blame you at all for resenting her.

If this were my YSS, here is the decision tree I would expect DH to follow:

Is YSS injured? If yes, call ET first and then me. He is her son and deserves to know if something happened to HIM first.

If no, then I expect to be called first because the damage is going to be to JOINT property between DH and I. DH can choose to tell ET later or have YSS do it.

ET is like your BM in that she contributes absolutely nothing. ET also lives an hour away. For a fender bender, ET can't and won't do anything. If YSS is hurt, DH can call her first, then call me, and I'd still beat her to the hospital with 30 minutes to spare.

Whether your SS should tell BM is moot. Your DH should have contacted the other person who was going to have to deal with SS and the fender bender, i.e. you. The order of operations changes when a BP decides not to be involved and abdicates their responsibility.

I feel you on this - your SO didn't need to lose his head calling BM in that moment unless she was going to help with something. 

SO will report to BM, when he could just deal with a SK issue himself, or bounce ideas off of me. Then at the end of the day, if he feels it's something she needs to know about, tell her then so she is simply informed. 

BM here will do the same, SO got a call around midnight during the work week a few years back, BM says YSS is at hospital. SO freaked right out until he learned that he had only sprained his arm & was now on the way home. The end. Of course, SO wants to know what's happening with SS's but within reason, text him so he knows when the checks his phone in the AM.  What is this - codependency? Drame Loving? I don't get it. 

After reading your history and the fact that BM does nothing and doesn't have any custody or pay any CS, i see no reason for your SO to have called her. What was the purpose of him calling her? It didn't affect her at all and SS wasn't injured. It's almost like your SO still wants to keep her involved for the pure sentimental reason that she's the mother of his children. To me, this is elevating her to a status she has not earned. She is a cowbird. 

My bat shit crazy XW was one to go ballistic against people who she rear ended.  To the point she would follow them to find out where they lived and then would cut their tires at night.  No recognition that if you rear end someone it is your fault.   

Your DH blaming some kid for walking through a parking lot and the person  in front of his idiot driver son slamming on the bakes to not hit the kid in the road, yeah, the idiot in this story is your 16yo SS and ... sadly.... his father.

As for DH calling his son's mother regarding the accident... meh.  Though I would not have made that call if the kid was not injured.  No need to contact an X over a non event.

IMHO of course.

I wanted to put my thoughts together and come back to this.  

You said the following: "

I happen to call SO while he was leaving to go to the school and he is all ranting and raving about SS16.. I can't believe this happened blah blah.

I proceeded to say (about 30 minutes later on a different phone call) that SS16 doesn't pay attention to shit... and naturally when you rear end someone it's your fault. 
 

He says how I like to cast blame etc and that some kid walked in front of the other kids car and SS16 didn't react fast enough to him breaking.. ok still his fault. 

I go ohhh ok so you m fing before you saw anything was cool and I can't say what I have too.

Then he tells me he called BM to let her know but... "I wouldn't understand that."

Here's the thing- sometimes as step-parents we get so blinded with our disdain, disgust, upset, or sometimes even hatred for a skid that it comes out in the most 'innocent' of comments.  The parents is typically VERY well aware of how we feel about their kids and take it personal-an attack on their kid is on attack on them.  That's just human nature.  It's how kids feel when the other parent talks bad about their mom/dad- they take it personal- because that parent is a part of them.

During throes of difficult times with a skid -even more so when there is a deadbeat parent involved (can be custodial or non-custodial)- we often participate in making the environment for the parent absolutely toxic with our comments, sighs, gestures, eyerolling, or body language.  I know because I did this regarding my own SD with DH many times.  Now, I wasn't extreme but I know I added to the dysfunction/toxic approach and made my DH more defensive or want to seek out/speak to anyone else BUT me regarding SD because he didn't TRUST ME (rightfully so) to give any support or kindness, but likely all negativity.  I worked on that part of myself hard.

SO, when his kid got into a fender bender- he was likely distraught, upset, even if it WAS his kids fault, he probably just wanted support, and the other parent can often BE that-EVEN if they are deadbeat, uninvolved, or otherwise a detached parent.  That deadbeat still has more love for the kid then the step does and won't automatically resort to trash talking them or making comments to further hurt the other parent, because in the end they have a bond with their child that puts love above all else and the love is unconditional (most of the times, not always).

My DH now is toxic this way with my oldest son, who has mental health issues.  His disdain and hatred is so clear, I rarely speak to him about ANYTHING related to my son anymore.  I rather speak to my ex (which I rarely ever do) -he moved overseas when things got tough for our son to never return (of course he had health issues to be addressed and then covid hit).  He is THE definition of deadbeat dad, and often comes down hard on him (he was an abusive ex/narcissist)- but even THEN he's more likely to not say as much negative shi* as my DH does.   Sad, my DH and my son were very close for many years, until the age of about 17 or so.  I have shut down completely and refuse to discuss anything about him with DH because I DO NOT TRUST him.  He is toxic-even if my son is as well at times, he IS my son that I gave birth to, and I will always love him even when I can sometimes hate his behavior, kwim?

So yeah, sometimes a parent would rather share things we see no reason for with the other parent because they feel they may get more support or they can empathize or 'come together' in sympathy or worry for their children, where your spouse will likely spout some smart as* negative comment (even if true) -as you eventually did.  

It's just another of dozens of reasons that blended families are so hard, so unnatural and often fail.  There is just not that mutual connection with the children, never will be.  It can range from uncomfortable to outright toxic.  Blended families suck for the kids, the step-parent AND the parent, AND the ex and spouse if they have one, it's just unnatural, it will always be 100 times harder then a nuclear/traditional home.  

So, accepting this, realize that there is no reason to be mad that he reached out for a more sympathetic ear than yours.  It's just the nature of the beast.   Please realize this is not condemnation, but just a reality that we sometimes forget.  My DH has done probably irreparable damage to our relationship with his feelings towards my son.

I worked HARD to fix mine with SD and made it happen-we are good now.   Then again, I damn well am a better person then my DH.  That's just the truth.   THis is part of the reason why our marriage is on terribly shaky grounds (amongst many other things).