Just because you get a flat doesn't mean it's a piece of junk, my car is fine and I've gotten flats and had to fix them. And if you are really unhappy with the car he's letting you use for free, go get a rental. I mean if I lent someone my car and something went wrong out of the ordinary, I'd pay. Flats though? Maybe you hit some nails or something. Anything you damage while using it is yours to fix. The battery was maybe a fluke, in which case yeah maybe he coulda paid. Or maybe you left the lights on. It's not like he lent it to you and then the transmission was going and the gauges were malfunctioning and other things that are not user error. Basically, I'm suspicious of most people and like they say, don't look a gift horse in the mouth. Instead of talking to him like a reasonable person, you're here complaining, so wither he's atrocious or you're wrong. We will never know.

Tell him what you've replaced and how much it costs and refuse to give it back until he repays you

Better to return the car before it dies on you and he makes you buy him a new car.

Seems it would be cheaper to rent a car

So your dating his daughter and you want to come off as a freeloader? If repairing is cheaper than a rental then be happy cuz he is not obligated to loan you a car. Or show some initiative and independence and get your own car.

why not pop all tires put a faulty battery in and cyphen the gas, illegal? maybe. stop a tool? possibly