I would have to say no.
I would say no becuase it would create wear & tear on the car & that'll cost you & not them more $$. If the car broke down, I'd have no way to help them.. but if they rented the car company will send a new rental asap to replace the broken one.

If they are low on funds & have honestly worked hard to save up the money, but just cannot get enough cash to rent a car, I'd help a little in the rental fees (like the down payment) & going there with them to make sure all is ok & the car is 100% insured.