If the prop is windmilling you have no control over where it might stop. In this case it stopped due to the reduced airspeed due to the raised nose as she flared and the rapid deceleration as they touched the water. If the prop has stopped then the engine has seized or your crankshaft has broken. So starter is irrelevant.
As I have advised her and all my other students, once you are committed to a forced landing the aeroplane belongs to the insurance company. No point trying to save a prop. I'm not sure in her case if it was developing any power, but I tell people to shut the engine down as a restart can increase the liklihood of a loss of control, we actually demonstrate this in stall and PFL training by applying full power from idle to an aircraft with full flap and trimmed for the glide.