Never had a problem with mates borrowing my cars, even last weekend a mate took my FRS to the ring, he daftly clipped a wheel getting off of the tunnel and then proceeded to beat himself up over it for the following 4 days.
End of the day it's a wheel and it happens, nothing a couple of days back at the refurbishes won't fix in the black they had already done and hardly worth loosing any sleep over (a £50 fix at most!)
i can definatley say he felt worse over it than I did.
my mums used my cars over the years aswell, she actually drives them as you should and never once had an issue with them.
she knows where the keys are if she ever needs one for a day