I think it'll be worth it, seeing most cars come with them for a reason. They do work. I previously had a Ford Taurus, and had mine removed and replaced with straight pipe when I had the exhaust system replaced. It wasn't more than a week later when I was back at the muffler shop because my car sounded more raspy and definitely more annoying from the resonating sound in the interior. Once it was installed again, it made the drive much more pleasant both in and outside of the car.
Also, do you know what kind of resonator your friend installed? If it was a "Cherry Bomb" or "Glass Pack" one, of course it won't reduce interior/exterior noise!