There is no love between DH and BM. He reacts very strongly and negatively to BM, from my memory. I would not mention it to him because I think it will only take the situation from bad to worse.
Anyone who listens to an ex talk about the parenting of the other has to realize it comes from a biased point of view. If your friends are taking BM seriously, I would have a low key discussion with them one on one, without DH. They should understand that BM is not rational in how she views things.
As someone who "knows" your BF, I think BM might have picked up on some of your DH's struggles with SS. I know you've said before DH wishes SS was more like the neices and nephews - not that DH has ever or would ever vocalize that thought - but I know the neices and nephews are athletic and not whiny and tentative like SS can be, and I know your DH admires them. If I know that, there's a good chance BM does, to.
If you thought it would do some good, you could mention to BM that you know she's been talking about DH and that for the good of SS she should stop. Just say BFs are difficult enough, and you want peace between them, and you don't want it getting back to DH, like it did to you, that she's been gossiping about your family. She really should stop, so if you think saying something to her will help, do it.
And, yes, your character has and continues to be tested, Phantom, and I think you are doing an amazing job!
ETA: I think this post is a testament to your character. I can imagine how infuriating it must be to hear that BM has been bad mouthing DH when you work so hard to keep things civil and there's no reason for her to be nasty. I don't know that I would have the presence of mind to not respond on the spot and make the situation worse.