What should i do 🤔

Not sure if this is the right group to vent to but I’ve been dating this guy for about 7 months and just found out he has a gf. I found her on ig and fb. I’m going back and forth on if i should tell her that he’s been cheating on her with me and send her screen shots OR just leave it alone and hopefully one day she’ll know what a piece of shit he is...