My friend has never used a $50 United Travel credit earned through a credit card promotion which is due to expire. Can he purchase a ticket partially using his travel credit (and additional funds) where I am listed as the passenger and he includes my United Mileage plus number? ...
If this is a TravelBank credit (where credit card travel credits normal go), Yes
If yes, does the value of the ticket become my property to use as I please?...
Yes
Does it make any difference as to whose credit card is used to pay for the balance? ...
No
In doing so is this transaction a violation of United rules, which seem to change frequently and are not as always clear cut? ....
No (as long as freely given and no compensation changed hands)