No offense, but, man, you're easy. A guy who's not one of your close friends -- just a friend of a friend -- walks up to you, hands you the keys to his old POS 911, and figures that entitles him to take your $70,000 Viper on an oh-sh*t! joyride. And you let him! I'd be careful, because if the word gets out, total strangers will be beating a path to your door with the keys to their Mustangs and Camaros and riced out Hondas looking for a chance to try their hand at wrestling the big snake around.
Seriously, you are very lucky that idiot didn't total your Viper and take along a couple of the neighbors cars with it. Most Viper owners won't let anyone drive their cars. And, as you see now, for good reason. (There was a message posted on this board a few months ago which featured a picture of a red GTS on its side down in a deep ravine and the title "My Dad wrecked my Viper!") Just chalk this up as a learning experience that fortunately did not have an unhappy ending. You are not being a **** for refusing to allow someone to drive your car.
I have let a few others drive my Viper. But I carefully choose the venue, I am always in the passenger seat, and the experience is preceded by an admonition against sudden applications of throttle with the wheels pointed anything but straight ahead. The guys I've let drive my car were just happy to have an opportunity to pilot the beast; ragging it or flogging it were the farthest things from their minds. I guess I'm easy, too...