In my opinion, yes. Geroge commits the ultimate act of friendship by not allowing the posse to torture and kill Lennie. Lennie could not have done it himself. He barely understands the what he has done, nor the ramifications that will truly follow. He knows people are angry and that he has hurt Curly's wife, but that is it. He still clings to the dream of escaping the inexplicabilty of society and going away with Lennie to "tend the rabbits."
Still, it is not an easy thing for Lennie to do. He needs George as much as George needs him and "his hand shook violently" as he held the gun, "but his face was set and his hand steadied." Aware of what will happen if he does not sacrifice his friend, Lennie is a true, but tragic, hero.