I think an anonymous letter/ email is the way to go. Just warning her to be vigilant if nothing else
This, and the entire thread, is nuts.
I had a very abusive H. 8 years later, I'm still dealing with the emotional and financial abuse I went through. Counselling is finally helping, as I felt totally broken.
We have 3 DC, and he's continued the abuse through them. My eldest doesn't see him at all, and he does as he pleases with the younger 2. Sees them when it suits. Has no practical role in their lives, it's a fun dad stuff.
He twists everything, is undependable & takes pleasure in letting me & kids down.
However he is in a relationship for several years now, with someone I know a bit. They are happy out, it would seem - they don't live together but are very involved. I've no doubt that I'm made to sound completely deranged, and he the victim. In terms of their relationship, all is wonderful, my guess is there are no expectations or pressure on him, she seems to mother him, and seems able to wilfully ignore his lack of engagement with the DC (I'm sure he portrays it as my fault in some way).
So while I 100% know what he's like, and no way has he changed, she's happy with this version.
I cough not warn her or get involved, even if I wanted to.
It's none of my business, would not be listened to, and anyway, is apparently not needed.
The idea that you would get involved in your ex's new relationship, because of how she looks (similar to you) & the speed of their relationship is really odd & in no way ok.
He is free to do what he wants, as is she. She is too. You have no way of knowing how this relationship will go, if she will experience the same situation. Of course he hasn't changed, so it's likely but is none of your business
I know my ex hasn't changed. He has put me through hell in our marriage & 8 years since. However, the dynamic of this relationship works for him. He instead continues to try & exercise emotional control over me, for who knows what reason. All that works is to ignore him as much as possible & have no expectations and not rely on him.