Ex & I have always had a relatively good relationship, he is my dc's dad and although he has at times behaved outrageously towards me I have tried my hardest to keep the relationship civil. We broke up around 5 years ago when our youngest child was 6 months old. As I say, over the years things have sometimes been difficult but overall I'd say we were quite good friends, we've spoken to each other about relationship issues etc and have been on evenings out together.
Last year he entered into a relationship with a woman (S) who he'd known vaguely for a few years. She has children the same age as mine who are at the same school, so I knew who she was but had never spoken to her. He seemed happy, I was happy for him and me & this woman actually became quite good friends, spending time together when the dc were at school, and we all went out together (with & without the children) a couple of times. At the start of this year, S had what she said was a one night stand with an old friend of hers, a man I know of and have met once or twice in the past. She didn't tell ex immediately, but then found out she was pregnant & had to come clean. She ended the pregnancy, ex forgave her & they seemed to carry on as normal.
Then last month S suggested we join a gym together, she went and visited a gym local to us (they live up the road from me) and we joined up. This was just before half term, so we didn't actually get chance to go as the children were off school. Half way through half term, ex, S, and her children went to visit a friend of hers a couple of hours away and weren't back when the dc went back to school. I went to the gym by myself, and soon realised that the man S had cheated on ex with worked at the gym. We got talking, like I said I did sort of know him, and I mentioned that ex & S were still away visiting her friend (just in passing conversation - I assumed he knew they'd stayed together). He looked a little confused, and later sent me a message on facebook saying that he had been seeing S since the start of the year, he had no idea S & ex were still together, and that she had been messaging him while they were away. He showed me one of the messages from her telling him how much she missed him.
This obviously came as a shock to me as well as this man, and he has since told me that he has made it clear to S that he wants nothing more to do with her, although I obviously don't know the exact conversations they've had. S knows that I know, as he told her. They're back from visiting her friend now, and have announced that they are thinking (seriously) about moving to the city they were staying in. I've barely seen S, only a couple of times on the school run and she is obviously very distant with me, and I with her.
I have no idea what to do. I feel like as ex & I have always been so close I should tell him, I know he'd believe me and I know that I wouldn't be made out to be trying to cause trouble. I am certain of this. But is it my place to say anything? Now this other man has (apparently) ended things with S should I just leave things be? I know ex would be heartbroken, but I also know he'd be incredibly hurt if he finds out in the future, and then finds out that I knew. Can I let him move away from his children with a woman who has betrayed him for 6 months of their relationship? Can anybody offer any advice? Because this is constantly on my mind and I don't know where to go from here.