If your friend cares about the money he gave away to the stealership, tell him to contact the dealer and tell them how much better price he can find. If they don't want to revise the price and refund some of his money, he can (at least threaten) to return it for a full refund as the law in your state permits. If the dealer wants to keep the sale, they'll haggle. BUT the key is that your friend needs to be serious about his threat, and willing to carry it out.
Maybe the GTP "find" is worth the money, but otherwise that'* what I would suggest. Shopping around can do wonders when it comes to car stealerships.
Current GMs: '95 Chevy Suburban C2500, '69 Pontiac Executive
Former GMs: '83 Buick Park Ave, '73 Buick Electra, '92 Chevy Caprice 9C1, '98 Pontiac Bonneville SLE, '99 Olds Aurora, '00 Buick LeSabre