Should i tell her or no?

Ok... So the other day i was at this dance thing for one of my friends. It was me and my best friend helping her out. The girl whos recital it was had let my friend hold her cell phone and she knew he was gonna text this one kid. And he did. We made this scumbag think he was talking to the girl but he wasnt. He baited the kid to say all this stuff like "i kinda like you, id kiss you if you did first" and he has a gf who i used to like and is one of my good friends. He never mentioned he had a gf either during the texting. I wanna tell his gf but i dont wanna come off like.. Im trying to ruin her reklationship so i can have her..Cuz im not. And her best friend doesnt wanna tell her becuz she thinks this girl doesnt listen and shell learn. Should i tell her her what her bf did or no? And how??