“Dating my “friends” ex”

So I was friends with this girl, we studied together and then become friends, over a year ago she started to get distant when she got a new boyfriend and wasn’t very interested in me or being friends which was fine I wasn’t gonna force her, we lost touch, I was diagnosed with cancer not long after and told her and she pretty much told me that it’s not her responsibility to be there for me and that she was busy with her new partner which fair enough, it isn’t her responsibility at all, since then we’ve spoke a few times, not much, just sort of hey how are you once every 3/4 months and it’s me that asks, she never asked about my treatment or what’s going on with me so anyway I’ve been seeing this new guy and he tells me yesterday he’s her ex, now I had no idea, I vaguely remember her telling me about him but I don’t remember anything else and I never saw photos of him from when they were together, just the name and it’s a popular name, so I said it was a bit odd for me but I’d think about it, so I chatted with her out of respect and she went mental at me calling me a whore, a slut, a bitch, a crap friend etc and I said if it’s an issue then I won’t do it but we aren’t friends anymore, we haven’t been for a long time and you’ve been with your other half for a long time now, and I tried to be as civil about it as possible but she attacked me, then started calling her ex attacking him. I wanted it to be easy and just wanted to see how she felt but her reaction was alarming. Clearly she isn’t over him is the bigger picture here but I don’t think that I shouldn’t be allowed to date him because she was once my friend? We don’t talk at all and haven’t for over a year but maybe I’m wrong. Am I? I really don’t know!