[MuGen]

It makes sense if he is her EX and not seeing him, but it's another thing to have sex with him, give him false hope, and go and sleep with someone else and on the first date.

If you are friends with her, and talk to her sometimes then I would leave it alone.
If you guys are good friends and share things with each other I would go up to her and ask her if she knew what she was doing? And if she knew what she was doing would have consequences? Then I'd tell her to stop the insanity or tell both guys that she's playing them. But then again you don't know if she's confused, or playing them.

Ask her if her feelings are genuine or if she's just messing around.