A friend change the wheels of my car. about 1200 miles later the front wheel came off. how ?

it came off whilst i was driving, and i am lucky to be alive. the problem is, it was his car, he was doing me a favour whilst my own car was off the road, and now he is demanding money to repair the damage to the car. I maintain that it was his negligence that caused the accident in the first place. am i on the moral high ground here, or should i pay what i consider is more than the car was worth ? Incidentally, i have told him he can keep my car, a rover 75 that needs a new gearbox which he was looking at repairing as he can at least sell it for spares or repairs, and will probably realise about the amount he wants from me, but he wants my car and for me to pay.