Luker, the stringer is a support beam that takes tension and compression. Filling it works in compression to some extent but not in tension.
It leaves you relying on the hull to take much of the load.
Your hull will flex more and fail sooner (but who knows how soon) and you run the risk of the driveline that the stringer supports dropping to the floor one day (but who knows how soon).
So I guess the worst case is that you have the girlfriend's parents with you when the engine rolls of the stringer and the hull splits in two at 35 mph.
Short answer is it probably won't help much but it might not be worse than what you already have.