I used to sell cars (not for long) so I have a little insight. If he wants to trade in, find out what he thinks his car is worth (reallistically). Then go to the dealer to negotiate a price on the new GT. DO NOT TELL THE DEALER ABOUT THE TRADE IN!!!!!!!!! After you agree on a price, tell the dealer "well, I'd like to do the deal now and not have to worry about selling my car. My car is worth X dollars. If you can give that, I will do the deal today." Then don't budge. They will tell you that the car has problems, bad tires, they are losing money, etc. Just don't budge from your position. Remember, even if you get the car at invoice, they are still making money on factory to dealer incentives and dealer holdback.
I did this little number on the salesman when I bought my Mountaineer. I didn't tell him about my trade until AFTER we had agreed on a price. The dude never even saw it coming. I had my financing lined up before I got there, and I bought the car for 1000 bucks less than the trade in value at the time. Because of this, I have NEVER been "upside down" in my car's trade in value/amount I owe. I also got 500 more than the trade in value of my old car in EXCELLENT condition (and that POS wouldn't hold oil for longer than an hour).
Anyway, if your buddy is serious about trading his car, this is the way to do it. And don't expect to get retail value for the trade in 'cuz it ain't gonna happen on a V6 mustang.