Isn't it also possible that FE doesn't fly often and thought, "Hey, I'm never gonna earn a free flight on United. Richard flies them a lot. I'll just enter his MP."

This falls into neither the Accidental-Goof nor the Maliciously-Hoping-For-1K-Treatment category.

If it were possible (and legit) for the passenger flying and the passenger whose MP number is used to be different, just think of what that would open up. People could ticket mileage runs that others would take. Want to earn miles for that cheap LAX-SYD fare but don't have the time? I'd give up mile earning opportunities for a free trip. It would be InsideFlyer/Starwood/Concorde all over again!

cnk