Well, she needs to be held accountable for her decisions. I think it's reasonable to tell this guy, "Look, I get that she likes hanging out with you. But if anything happens to her while she's with you, you're gonna be held responsible." Do not threaten him. Just give him that polite warning. However, I think you should just call her bluff and break up with her. If something isn't already happening, something probably will be. A girlfriend doesn't ditch her boyfriend for a platonic friend.