Yea do yourself a favor, get ALL the info before passing judgment on a poor dude that is getting lead along just as badly as you may be. She MAY be doing one thing while your around, and something completly different when your gone. Get all the facts first. Ask to see her cell records, see if she is calling or texting him. If she refuses to give them up, I would assume guilt right then.
Good luck brother, there wont be a happy ending.