All I can say is that I sincerely hope she isn't telling your SS that he's going to die in his teens ...

Well ... you did start the drama, I don’t blame your DH for being upset. Why do you even spend time with BM if you hate her so much? 

You aren’t going to change her with criticism.  How would you feel if she asked you a snarky question? 

Just stay away from her. 

What good does it do your DH to call BM out when he knows she is lying? Especially in public?

My DH knows BM lies. He has the expectation that she'll lie, manipulate, sweet talk, etc. He doesn't expect better from her, so he acts accordingly to keep their conversations light. If it isn't going to matter in 5 minutes, my DH isn't likely to put more than 5 seconds into it.

In this situation, BM got passively called out by the doc. You and DH know the truth. BM has now been td the truth. Your DH can now show SS the truth. So calling out BM does nothing but stir the pot.

I get wanting to hold people accountable for their actions, but this is a situation where the people who should have called out BM were the doc, SS, and DH. Not you. That's why your DH is upset - because you know she is full of it, as does he, and you know she'll blow her top, and calling her out wasn't going to do any good, yet you did it anyway.

His standard of her is probably one that he expects her to function. He expects you to shine, because he loves you and married you for your good qualities. He expects better from you, because if you pull the same BS as BM or cause him the same amount of drama as she did, there is no reason to stay married to you. He's stuck with BM until his kids age out, and he's going to do things to make his life easier with her (hopefully without compromising his boundaries) because he wants his interactions with her to be few and far between. Don't take his apathetic kindness towards her to be feelings of goodwill. It's to keep his relationship with her drama-free, nothing more.

I agree with the others... you know she is a liar. Just document it and move on. Make sure that SS doesn't think he is going to die!! But other than that calling her out in public will only ever cause a scene. Your DH can do that privately through email or text- always keep things written, but verbally anything anywhere coming from you will only lead to a disaster. 

I'm more interested in why you provoked an argument at a stressful time than why she may have said the kid's condition was fatal.

If you really think BM's mentally ill, why would you even ask why she said it?

Maybe another doctor told her something that made her believe the kid would die. Maybe she said it was fatal to get your husband to take it seriously. Given that you heard this through another person, maybe she didn't even say it was fatal. Maybe she's so distraught, she's reeling and making a catastrophe out of this, but given that she's dealing with her son's recent diagnosis, can't you give her a break?

I think your husband is right. You caused unnecessary drama at an already tense time. Who cares why she said it, if she said it? It's her kid and she's probably worried sick about him. Maybe she's nutty as a fruitcake. Do you think your husband needed to deal with you two cat fighting when he was dealing with a sick child? You didn't even think about him, did you?

Either way, challenging her to explain it was unnecessary. You actually took a situation about their sick kid and made it about yourself. Ask yourself what you HONESTLY wanted to get out of asking a question you knew would provoke a fight. Maybe you like the drama? Maybe you're trying to prove your point to your husband? Were you jealous that his "first family" was getting attention?

There are times when we should bite our tongues for the good of everyone, even people we don't like, especially if there's nothing material to be gained by saying what we want. This was one of those times.

You'd be surprised at how many people who get a maybe diagnoses that run to the internet and scare their self to pieces with the worst of the worse scenario and continue to read all the rare but possible out comes. 

You seriously can't fault a mother (or father) for freaking out and/or fearing the unknown when it comes to their children. Being that both mother and father were attending the specialist consult , it's not like BM was going to somehow keep what they both were told from your husband when he is sitting right there hearing the exact same words. 

Frankly, you were the tag-along. Some mothers (unfortunately) would  have refused to allow you to attend the consult... but this woman agreed to you coming and agreed to have lunch with you. And how do you behave yourself in return? You call her out basically attacking her. Not cool. Nope, not cool at all. 

I can't blame your DH for ending the lunch and ushering you out. And I wouldn't blame BM if in the future she rejects your presence at drs/hospital interactions. 

And for the child, I suggest your DH see about a support group in the children's hospital to get his young son involved to see and learn from other children on how to work with and deal with his particular blood disorder. These groups are geared towards helping children learn to live with their disease to be as normal and full of life as they can along with coping with any limitations they may face. 

I agree with the others.  You definitely stirred the pot of sh!t.  My mom always said, "S/he who stirs the pot should have to lick the spoon."  

Why cause so much drama, especially in public?  Totally unnecessary and now you are licking the spoon.  

The wording probably put her on defensive.

"What made you initially think this was fatal?" Gets the same answer... Less confrontational than what was probably taken as "why do you always have to exaggerate????"