This is a point I forgot to mention when I previously posted . OP becoming involved with her friend’s ex puts her friend in a really uncomfortable position few ppl are put in - the possibility of really private and personal information being shared by a partner with someone they know. That could be sexual matters - “X never did that, she said it was demeaning” or “she wasn’t very good at it” etc, or maybe personal insecurities and secrets, or family information, that she only shares with very limited ppl who are extremely close to her.
It’s a horrible place to be knowing that intimate details about you can be shared once the relationship, which you may feel will go on forever, comes to an end. But usually you don’t have to think about it too much bc you don’t know the person they move onto, and have nothing to do with either of them again. Even if OP never hears from her friend again once she starts a fling/relationship with her ex, her friend will know that she’s not some anonymous woman her ex will be speaking about/ laughing about/ deriding, but someone the OP actually knows - and it works both ways - her friend could also be fearing OP might share some private information with her ex that she wanted to remain undisclosed - information she never in a million years would suspect he’d be privy to after they’d broken up (he dumped her). She may worry OP might share the newly-learned gossip information among the wider friendship group. Horrendous.
She really is in a vulnerable position for all sorts of reasons and she has a wedding she’s no doubt looking forward to/ stressing over and you’re adding more worry onto this now aren’t you.
You’ve likely made up your mind, OP but if you’re going to plough ahead with it, admit you’re being selfish and don’t give a shit how your friend will be affected by this - it could impact and potentially damage her mental health. She may be worried about her upcoming wedding and whether to invite you or not in case you bring him - the impact it will have on your friendship group and her support network…. Fair enough, this isn’t your problem, but do you have any empathy towards her? Are you fond of her or is she more of an acquaintance?
Saying OP shouldn’t be dictated to by her friend is understandable, particularly when said friend is engaged and has seemingly moved on, but there is personal history here and a unique set of circumstances- primary being she didn’t end the relationship, or if she did it was involuntarily, bc her ex wouldn’t move in with her, thus drawing a line under their relationship by his lack of commitment. She didn’t want the relationship to end. OP also asked her how she felt about it - which if she ignores is spitting in her face basically. She didn’t have to ask, but I suspect she was expecting hoping for her blessing so she could pursue this guy guilt-free.
Going out with a friend’s ex is of course a matter of choice and isn’t strictly forbidden - but it’s similar to sleeping with an older friend’s son or daughter - if it’s legal and consensual, there’s nothing wrong with it, but it’s still not morally right and is being inconsiderate to your friend’s feelings, which from this thread it sounds like many ppl are happy to ignore for their own personal gratification.