The guy who sold the car has taken a hit no pun intended on its value through no fault of his own. He had an asset worth x amount and sold it for y amount which is less so why shouldn't he receive the estimated difference from the party that caused the damage in this case $1800.00
None of us know this.
As he sold the vehicle, he now has to prove he continues to suffer a loss caused by the damage.
What if he sold it and took off only $300 for the damage. Or $0? The law can only restore a person back to the state they were in prior to the event. You might not like it, but that's the law of equity.
So, for those of you saying he is owed the full 1800 - he is no longer owed it. He was never owed it. He was owed repairs to his vehicle. She could have found a dodgy panel beater who offered to do it for a slab.
If he didn't agree to the cheap repair, he could have sought his own quotes, and if she refused, they could go to court and argue what the actual damage was and what the actual cost was that she should pay.
She got lucky here. But as I said earlier, an hour with a solicitor will tell you your chances of fighting this. Get them to write him a letter stating your legal position as you see it.
I reckon your chances of avoiding any payment are very good. Would love to hear the outcome.