advice please...

Okay so I have this guy friend and him and I have been best friends for this entire school year but the only issue is... his girlfriend doesn't want us being friends or even communicating with each other anymore... it hurts.. because he is the only true friend I have. He's currently trying to convince her to let us stay friends but she just won't budge... and I don't want to hurt their relationship... I'm thinking of telling him that for the sake of their relationship that we can only be the type of friends that smile at each other in the halls as we pass and nothing else.. but just thinking of that hurts me so bad... I don't want to lose him as a friend but I don't want to potentially ruin his relationship... I'm just stuck and I end up crying every time I think about it... I don't want to lose him but I don't want to hurt him... I wouldn't do this for his girlfriend either... I'd do it for him... I just don't know... any advice..?