The best car disabling device is called cash on pickup.
He creates his own problems by trying to help. Not encouraging people to save up first actually creates the problem in the first place. One of the
wealthiest ordinary people I know fixes cars for cash only... and his customers are happy as well. If they can't pay, the vehicle stays where it
Helping people who can't pay and then moaning they don't pay is plain stupidity on the part of your friend.