and don't be mad at the guy, you'll be letting the girl off too easy. your concern is with her not him, he is nothing but an obstacle. And i see no problem with him being there, you should allow your girl to go out friends if they are guys, you should be able to trust them like that. She obviously just slipped up. as i said before your call if you want to handle the stress of thinking she might cheat again.
Even still man, you're 18...so don't get too caught up. You've got a lot more time.