Errr, i would remind your friend that you dont wish to be mean, but that it seems like he likes this other girl at school too, and that it doesnt seem very nice to act this way to this girl when he is sposed to be comitted to his gf.
I dont think you should get a fight with it, just say that if you were his gf, you would be really upset to find out that he was touching up this other girl, and that you are only telling him because you are happy for him and his gf and wants them to be happy.
I dont know, its tricky, but if he does not stop with this after someone tells him that he shouldnt be acting liek this, maybe you should just let his gf know. That waya t least she would know, and she wouldnt feel so huumiliated or left in the dark :/.
I know its a tough situation but maybe just meniton it you your friend, and then if he doesnt stop, all you can do is let his gf know, and leave them too it. Your friedn may not appreciate you constantly checking up on what he is doing, but at least if you know that his gf knows, you know that you have done something to try and help.