if that was my friend i would go to the gf and seriously talk to her that if she isnt serious she should stop playing with my friend and stop cheating because i wont stand by and let my friend get hurt..
Talk about it with them. Not him, not her, the two of them. Dont say she s cheating on him, only say that it seems that way and why. Above all, be very calm and its good to have people to support you. Gby