Me (19m) and my friend (19f) had a falling out over her ex

Hi.

The title sums up my topic but I'll give a short explanation. My friend (19f) got cheated on by her ex (17m) almost 1,5 years ago. Over that time I've learned a lot about that relationship including the fact that the ex manipulated and assaulted her/forced her into things. In addition, she has been very against the idea of me going back to my ex who assaulted me and cheated on me.Now a few days ago I learned that they're giving it another chance. I tried talking to her but she's pretty certain he has changed. I told her that after everything I learned about how the ex treated her, I can't support that. This basically turned into an ultimatum for her. I don't want to make her choose. I Frankly don't know what to do in this situation. Does anyone have any advice?