I'd eat the arm. If I were to be slowly and excruciatingly painfully killed, and I knew it would happen to my friend(EDIT: and if I knew it would happen to my friend unless he hate my arm), then I'd sure as hell not want my friend to have to suffer it either.
I have nothing against eating a dead person, and even killing the person is okay if it's the only way anyone's going to survive at all, and especially if he himself is going to die soon anyway.
"How is the survival of one more, but death of all in a few days, better than a chance of complete survival of the rest?"