Is there a rule not to have a relationship with your best friend's ex-boyfriend?
It didn't happen to me but I don't think I will be comfortable hanging out with my best friend with my ex-boyfriend as her new boyfriend tagging along. It will just bring in memories. Even if you have moved on completely, I can't say to my friend "I am happy for you two" when at the back of my mind I am really saying "It should be me". I will be too paranoid to think that he's maybe telling my best friend that she is better than I was when we were together. I think it will be awkward too for my best friend because she knows our better days.
When it happens to you, how are you going to deal with it? Would you still be friends with your best friend? Would you rather slowly drift away?