You gave an SLS AMG to a friend of you and he no longer has the game?? That's the stroke of a genius. *sarcasm off*
As long as your friend still got the savegame it's no problem. Go to him with your gt5 disc and start the game with his psn account. He can load the savegame with your blueray and send you the car back, as he will no longer need it.
If he live's quite a bit away and going for a visit is to much of a problem ask him if he would be nice enough to give you his psn account data + the savegame. Make a new user on your ps3, put in his psn data and load the savegame. Voila send yourself the car back and all is fine.