I'm not really sure where to post this but anyway....got myself in a bit of a tricky situation.
Been with dp for 18 months now, I have 2 kids, he has a son. Was with dp for about 9 months before introducing our kids to each other.
When dp thought the time was right for me to meet his son, dps ex wanted to meet me which was absolutely fine. They had an agreement in place that when they find new partners, they would introduce to the other parent first. No problem with this at all. Apart from she made it very hard, she pretty much interviewed me lol. Asked about my criminal background - not that I have one - have I ever done this or that. I've never been so anxious in my life, and it frustrated me to the absolute max as I have 2 children myself both older than ds son and she was acting like I didn't know how to be around children.
Anyway a day later she pretty much said I'd passed the test 
and my kids and dp and his son are one very happy family :-)
Now last week his ex mentioned to do that she had started seeing someone new, but basically saying for now they were just friends with benefits - trying to show off I think - and it would take a long long time for it to become serious and so dp had nothing to worry about anyone being around his son. Dp was pretty puzzled as to why she was telling him this and so was I to be honest.
Anyway fast forward to this morning and I decided to join instagram, never used it before but starting my own business page. As I was following people, I decided to be nosey and search for dps ex. Found her straight away and her profile is all public. Turns out the person she is seeing has met dps son as there is pictures of them on her instagram. Even with the hashtag #bestfriends under one picture. It's all been in the last couple of weeks. Last night they all went trick or treating together.
Now my question is do I tell dp? He will be absolutely furious when he finds out. He was always very serious about the original agreement they had and even when his ex made me feel like absolute shit, we still all stuck to the deal. I just know it's going to cause so much drama. Why she is posting picture of them I've got no idea, dp isn't on instagram and doesn't use fb much. He moved to the next town when they split so I'm guessing she thinks she's safe.
What would you do? I just don't want to be the one that starts all this. I feel it's not my place and it's between them both. But then I'm so bloody annoyed at what she's done. She's only known him 3 weeks :-( dp will not be happy if he finds out.