I know of 3 specific cases in which a diver signaled an intention to end the dive early, returned to the surface alone, and died along the way. I once had to end a dive very early because of a bubbling hose, but in that case it was okay to let me go alone because I was so very close to the beginning of the dive and the reason I was ending it was obvious and not related to health. If there is any hint that the diver may have trouble on exit, someone needs to be with him or her.