Some string material degrades more quickly with time. Sometimes strings wear very rapidly because of poor construction or because it is rubbing on a cam. Without looking at the string, no one can tell you for sure if it needs replacing. Why not ask them why it needs replacing? Then base your decision on the reasonableness of their answer. Keep in mind that it's better to be safe than sorry, when it comes to the chance of a bow coming apart at full draw - trust me, I know!

I replace mine every year, but then again, I build my own and I shoot a whole lot more than 300 shots each year. And, I never wait until I see noticable wear.