Um, if a car hydroplanes, it IS the fault of the driver.
It doesn't matter whether you are in the UK, Canada, Germany, wherever, by law the driver is supposed to maintain control, and an accident caused by hydro-planing is an at-fault accident, whether the police issue a ticket or not.
If the car rolled, it is likely a fairly expensive repair, he needs to claim on his insurance, that is what it is for.
Claims for whiplash aren't always easy to make. If he has full comprehensive AND collision and upset, he will only be out his deductible (excess?).
He will need to file a report with his insurance, or your claim for whiplash will generate an investigation anyway.