People who let others sit on their cars are not car enthusiasts. Period. Seriously, who lets someone sit on their car and risk even micro-scratches from jeans or even worse damage from buckles, etc?
Hell, I don't even TOUCH the surface of my car with my fingers EVER let alone sit on it.
Anyway patch it? Seriously? I hope you mean professionally and by a body shop.
Whats the downside of the crack spreading? The fender would need to be replaced? It needs to be replaced already.