What should my friend do? His girlfriend doesn't even believe him!?

My friend is having issues with his girlfriend. I care about him so that's why I'm trying to help him. Anyways what's happening is there's this one other dude who has a crush on his girlfriend and is trying to steal her away by manipulating. He got in an argument with him online then the dude screened only one part of it, showed it to my friend's girlfriend, and made it seem like my friend was being the mean one. ut behind her back, that dude is very mean but now that he tricked her with the screenshot, she won't believe me or her boyfriend(my friend). She believes this bad *** instead (who's secretly trying to steal her away and won't tell her). He's so tricky! I can't believe she doesn't believe her own boyfriend, and he's so nice to her too. What should we do? He wants me to help him save the relationship.