Because in the OP's situation, the friend's reservation was made on-line using the friend's Aadvantage account number and thus already preexisted in the system. IMO, this is a simple case of agent error.

I had a similar situation occur a couple of years ago. Called the EXP desk to book an international itinerary using a partner airline for my sister using her Aadvantage account. Gave the agent all of the relevant information and of course it was ticketed using my miles.

Called back to remedy the situation and all was taken care of and no additional fees were charged.