It starts and idles? Then it is most likely not the battery (or the starter or the ECU). Is this carb'd or FI?

I would start with the fuel filter.

Then I would consider plugs, wires, cap and rotor.

These engines are relatively simple. If something is wrong, they usually will not start. Most of the time, addressing the simple things will fix the problem.

When he says it will not accelerate, does it stall if he tries or does it just putter along gaining speed slowly?

Good Luck,
Michael