I'm sure it was a pricey one when new! If I was into a convertible and the everything else about the car, I'd say your friend's 15,500 offer isn't too bad. I just don't know how popular a convertible MT82 Coyote stang is to the majority of the Mustang market, it might take a while at the price mentioned and possibly a long wait for less than mentioned to the general market, but that's a guess. In my opinion, the best way to value a vehicle is do a search on autotrader for a similar year car within 500 miles of your area, and see what others are asking for a like car. If you like it, and it's what you're wanting from a Mustang, I'd say buy it from your friend rather than giving your money to some schmuck from a dealer who maybe gave 11K for the same car on trade and is now asking 19.
Right now I'm considering a very well priced, low mileage, standard cab, short-bed 2014 4WD F-150 local to me, (about 4K under NADA), but I'm scared to take the plunge because it's a standard cab, and I know in about a decade I'll probably have to pay someone to take it away when I'm ready to trade up because everyone wants a 4 door, or at the very least an extended cab.