Got myself in a really messy situation

Hey all! I am really confused and could honestly do with some advice. I was dating a guy for a couple months. From the beginning, he said he was seeing someone else and I decided not to ask more questions. After some weeks he said he thought we should only be friends, I agreed since I do appreciate him as a person and did not want the split to be awkward. Just a couple weekends ago I met a friend, a really good one, I had not seen in a while and he mentioned he had this new boyfriend he was super into and that he wanted us to all to meet. When he showed me a picture I wanted to die since it is the guy I used to date. I have no idea what to do. How to address this or even if I should speak at all.The new boyfriend of my friend is a great person and although I am truly sad it did not work for both of us I am genuinely happy it seems to work for them.I kind of stalked their social media and realized it was there all along and I just really did not realize (not really an FB user). I don't think they know I know, although if we meet in person this will be a disaster, what do you all think I should do? I don't want to lose a friend but have no idea how to act...