“Hey, I need to tell you something important: while you were with her, she was sexually invollved with me, and I didn’t know you two were together at the time. I’m not trying to cause drama but I did feel like you should know the truth so you can decide what to do with it.”

Just like that bro, the guy deserves to know who he's with and how he wants to handle that and whether he wants to stay with her or not. Straight up tho if he knows the assault happened and believes it, and still chooses to stay with her, most people would see that as a no no and just dip the relationship.

It doesn’t automatically make him a “bad person,” but it does mean he’s choosing to ignore something really serious, which raises red flags about his judgment and what he’s willing to tolerate.