There are illnesses that cause people to sleep when they would not normally be asleep. If you boss knows that this person has such an illness and has agreed to a workplace accommodation for it, then he would not necessarily be at liberty to discuss this person's health issues with the rest of the team just as he might not be allowed to discuss any personal issues you have with others. If the boss knows about the sleeping, and in this case he clearly does, then let it lie. Give the person the benefit of the doubt that he is ill and continue with your own work.