Ok, here's the thing:  You cannot care more than the parents.  I get it, it's frustrating.  I've been there.  My OSD then 12/13 was all over all media 24/7. She missed sleep, she didn't do anything beyond her phone/computer when she was with us.  She knew how to get around things and I'm sure had differnt accounts - but really, it didn't matter. I could cajole, yell, ask, plead, suggest and give warnings about online use and teens but NOTHING I said helped - because the parents didn't set limits.

DH checked her online logns ONCE that I know of. He said, "It's nothing bad, not a problem." This was via our provider, not her phone BTW. If he searched her phone or computer he believed it was an "invasion of her privacy." He tired setting wifi limits, bedtime limits, to no avail (try = once or twice). He would NEVER take her electronics from her ever ever. He asked me to pick up a timer so that the wifi / router would turn off / on at set times. It's still in plastic in our storage area, never used.

I would tell him once, and once only about your reservations and what you have observed then leave it for her parents to deal with.

I would let him know that I accidentally discovered the account and let him know I was blocked before I could show him.  And that would be that.  He then knows about it.  He can do what he will.  You have told him what you know, so you can never be accused (rightfully, at least) of keeping it from him.  If by some chance he's one of the clueless few who has no idea of the trouble teens can get into on social media, you could let him know, or direct him somewhere to read about, the risks. And then you're done.  You don't have to ask him what he's going to do about it or think about it again. 

I agree that there should be limits on a child's access to social media and the internet.  I think parents should know the passwords and be in a position to check on a child's social media from time to time.  They often need protection from themselves, and have no idea how dangerous some of what they do online is, or how long the garbage they post stays out there and how it may affect them in the future.  But ultimately it's up to the child's parents, and not your problem. 

Make sure your husband understands that teens are extremely stupid and share nudes that could get them charged with child pornography and have to register as a sex offender.   Long term consequences for a moment of "being cool".  
 

After that it's on him.  Sounds like a typical dad around here, denial is not a parenting plan. 

I tried warning DH about SS posting like topless and sexualized photos. BM thinks it's fine cause she's trying to be his bestie and DH is a lot of talk but no action, so nobody watches his activity. Before I disengaged, I tried to but I got banned from SS's IG. His Twitter is something random and I'm not sure what other types of accounts he has. Them not caring is a big reason I nachoed out. 
You can't care more than the bios do. You will always lose. The best you can do is to tell DH what you see, give the warning of the potential legal ramifications and move on. It'll drive you nuts otherwise.

I would make sure the parents know and understand what is going on.  

My approach on how to handle it would be different.  Kids are smart and tech savvy.  If you bust her on this account she will get another one.   If you take her phone, block data whatever she can use computers at school or borrow phones etc..  Busting her pushes her farther to the dark side of hiding and God knows what lurks there.  I would wait a little and make a phony account and friend her, follow her whatever the site is.  Then you can see the truth, not the truth she wants to share with you.