If they paid full price for it, send it back to them and ask them to purchase the unlocked version. It would be the same price.
It is possible to pay $$$ to get them unlocked, but I don't think $150 is worth it if the phone is paid in full. Now, if they got the phone on subsidy so that they only paid $200 for it or so, then the unlock fee may be worth it.
If it was purchased at an Apple store, you may be able to do the swap yourself. If it was purchased at an AT&T store, then you'll likely need to be the account holder it was bought with in order to return it.