We are very over btw, I'm remarried and he is due to get married soon too, but for reference we have DCs and shared custody of them. When we were together there were all manner of issues, but the most commonly occurring was that he likes to get in contact with lots of other women on OLD to bolster his self esteem.
I saw my cousin-in-law this weekend and she mentioned she had been in contact with someone who had the same surname as my DC, wondering if they were related. And lo and behold, it turns out it's my exH.
Now I'm kind of torn on what to do. Obviously the right answer here is that it's none of my business and to stay out of it. Except a messy break up down the line is my business in a way, as it will affect my DCs. Then I also feel uncomfortable not telling, as 1) I felt like I was going crazy for years and would have appreciated "proof" from someone, and 2) I feel that I've achieved something in working through the shame of having this secret (him cheating on me) that I never told anyone about for years, and now I feel like I've been thrown back into it against my will.
I'm angry that I can't win here, that I now have this burden of knowing about his dodgy behaviour and no way to deal with it. I know telling his DF is probably the wrong thing to do, and if she's like me she may well know already anyway. But I don't think doing nothing is fair on either me (the burden of knowing there is a ticking bomb) or her.
There is also an underlying issue here, that she is much younger than him (afaik she wasn't the OW, but I know he did know her before we separated), and he has controlling tendencies. So I'm worried for her 😕