Why do people behave this way?

My bf and I have been together a year. He dated a girl a few years ago for 8 years. They have a lot of common friends. They broke up because he found out that she was cheating on him/was taking drugs/living a double life. She started dating a new guy a month after they broke up. He was devastated. At the beginning all their common friends took his side. Two years later when he started dating me, she asked if they could all be friends again (what a coincidence?). The common friends funnily enough do not support our relationship at all (e.g they always like her photos of her and her bf while they never like the ones he posts of us and like the ones that I am not in). They act as if I am the reason they broke up/prefer if they were back together. It kind of bothers me even if it shouldn't. My question is why are they behaving this way and not being welcoming like they are with her bf (when she at the end of the day acted badly)?