Make sure that the gas is brand new. Even if she ran the mower until it was dry at the end of last season, there can still be old gas in the carburetor bowl. If draining the tank and then putting in new gas doesn't work, take the carburetor bowl off, drain it, clean it, put it back on, and then try it again. You won't need to take the carburetor off. Remove the plug to see if you have carbon between the electrodes. This carbon can build up to the point where the gap disappears completely. If you have a gap, though, you should be okay. Clean the air filter. As a last resort, you can use starting fluid _conservatively_, and understand that this might do more harm than good, if you have not done everything else possible first. If all else fails, move to Austin, and go into the music business. Just kidding.
Mike Mandaville