You need to seek legal advice for this. Your girlfriend's mother drove the vehicle without your permission. As a result, your girlfriend's mom is ultimately responsible to you for whatever damages you incur.

Your scenerio implies that U-Haul did not list her as a driver for the vehicle. As a result, U-Haul will hold you responsible for this accident. Now, if you had given her permission to drive the vehicle, the insurance company may have covered her. However, without your permission, chances are your insurance company will not get involved - which makes this your problem.

If you purchased additional insurance from U-Haul, you will need to seek advice from that agent.