I'm sorry I beg to differ with #2 above. As long as you are comfortable / trusting of his luggage contents and that he's not having you transport illicit goods for him, this is no different than family members consolidating baggage under the member with highest privileges.
+1. Although checking someone else's bag under one's own itinerary probably breaks United's rules, it does not reach the level of an ethical dilemma.
I've checked traveling companions' bags as my own. I've also packed friends' and family members' possessions in my bags. As long as you're willing to answer for a bag and its contents, I don't see a problem here.