I'm sorry I beg to differ with #2 above. You and your co-worker are traveling companions on a flight. 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. Or at least, not any different that UA needs to know.
You step out of the taxi holding colleague's bag. What is UA to know the difference?