The tire could be fine, I've had tons of cars that people have driven on the tire flat and the tire be okay, what makes it go bad is not IF you drive on it flat but HOW FAR you drive on it flat. Take it to a tire shop and have them look inside and if its bad you'll see tons of shaved rubber from the inside of the sidewall. The longer you've driven on it the thinner the sidewall is and the worse condition of the tire. A little shavings is okay, its the sidewall your worried about.