Is it normal to not work on your wife's cousins fathers car

I few years back my father did work on his vehicle and did not pay in full. My father warned me about that family's sketchy ways. So when he came to the shop he was asking if I could do him a favor and fix his car. I feel like he has some balls to come up to someones shop where he owes money and ask for free work done to his car. Or am I wrong here for blowing him off? My dad told me to stay away from those types of people even though they are some what family.