Likely the real sequence of events was: They tried to start it and the key sheared then the engine backfired.
If the correct technique is not used when placing the flywheel and key onto the crankshaft taper and keyway, and the flywheel retainer nut is not torqued to 100 ft lb, that is what usually happens.
Place the flywheel onto the crankshaft taper, aligning the keyways of both. Then, very gently work the key into the keyway until it is flush with the surface of the flywheel. Installed flywheel nut and torque it to 100 ft lb. Do not put any type of lubricant on the taper seat of the crank or the flywheel. Use only a genuine Briggs key.

Keys shear for a couple of different reasons however based upon the engine repair history Mownie is correct here. Believe it or not older people fall because their hip just broke, not fell then broke their hip. Double check valve lash upon assembly too.