checking her phone is a wack move straight up.
having said that...
i don't even blame the other dude. he's just a lame doing what lames do.
your girl is probably not cheating right now. she may have slept with him before you were together. or maybe he ate her out/she sucked him off etc.
your girl either knows he wants to hit and she's allowing him to flirt (and by the same token allows him to completely disrespect you and your relationship with no consequences) OR...
Your girl is truly clueless and as dumb as a sack of rocks/airhead status. either way, do you want to be with her?
I never feared other dudes trying to flirt with my girl. I don't trust them but at the end of the day you can't stop a snake from trying to bite. however it's always up to the girl is she wants to accept his advance or keep giving him hope. that's the only thing i'd watch out for. and if she does, you just reprimand her/dump her without even mentioning the other dude's name (cause it's not even about him. it's about her disrespectful ass).