Cheaters

So I know this isn't really for eve but I don't have any females to go to about this.

So my best friend started talking to this guy and they've been talking for a little while but we just found out he has a girlfriend. Obviously she's done with him but we don't know how to let his girlfriend know he's been cheating on her and played on going on a date with my best friend. (Unless she knows of course) We planned on inviting her to the "friendsmas" event that was to be my best friends and his first date together but I don't know when to let her know or even how to approach it...

Please help ?