This guy got someone to put a butt in his face when he was sleeping at school, and I feel bad for doing nothing?

Ok so here is this round guy at school who plays games all night and is really tired during school. He slept between 2 classes when we had a break. He was sitting, leaning against a wall just behind him. One of my clever friends said: Hey, this guy is sleeping!

Another of my friends said: Hey, D! Could you come here and put your butt in his face?

So D came and did it. And what did I do? Absolutely, nothing. I walked away, but i didnt say anything, i didnt smile, i didnt laugh, cry or do anything else. But the worst thing of all...was that I didnt stand up for him. How do I stand up for guys in these situations? I dont care what happens with me i just dont want this bullcrap to happen. I see these people taking pictures of this guy almost everyday, just cause he is a bit round. The thing is, they dont understand that this guy has had an illness that prevents this guy from losing all of his fat and makes it really hard. He told me that once.