I drove a '07 Mustang GT 4,6 Convertible the other day, and I must admit it was somehow a very enjoyable car.
First of all though, like most american musclecars, it's crap. Technically it doesn't stack up to a modern european car, and the same can be said about the interior, which mostly reminded me of a Kia Picanto (even though the Mustang had red "leather"-seats). The dynamics of the car are...well...non-existant really, and the comfort isn't that great either. But of course, all those things are not important. A lazy american musclecar is all about cruising down a long, straight open road, listening to - preferably - the V8 in the front. Yummy!
We don't have mountains in Denmark, so our roads are actually perfectly suitable for a Mustang, Camaro or whatever. The problem is, that long, wide and straight roads are otherwise pretty rare in many other regions of Europe, also in Serbia I guess. The size of the car could become an issue in the long run.
I don't think a Mustang would work as a car to use every day in most european countries. It's just too vulgar and thirsty and...american. Then there's the price. Someone mentioned that you could get a slightly used Mustang for 30K USD in Americaland. In comparison, the car I got to drive had about 30.000 miles on the clock, and still you'd have to pay just over 100.000 (!) dollars to get your hands on it on danish plates. I doubt it's that expensive in Serbia, but frankly, if your friend isn't interested in raw muscle-power anyways, there are a LOT of better alternatives.