I am guessing that they terminated him for not reporting an accident that he should have had knowledge about.
The key words are "should have had knowledge about."
They will match up the vehicle and the damage and show that his vehicle did the damage. If they cannot prove it, he will be back with backpay.
If his vehicle matches up with the damage, marks on his vehicle also, they can fire him for an unreported accident.
But now they have to prove that he should have had knowledge of it. If it was a minor dent or scrape, it is very possible that the driver did not know he did it.
Should be back with backpay.
If half the trailer was taken out or some other kind of major damage, UPS will say that the driver should have known about it. It does not matter that the driver denied doing it or not.
If there is compelling evidence that he did it, and a typical driver would have known he did it, then it could get ugly.
Sounds like it was somewhat minor and it is very likely that the driver had no idea he did it. Should be back with backpay, or at least a mini unpaid vacation.
Since he already denied doing it, it he did do it and fesses up now, it could get ugly as well. He may not come back because he now has 2 cardinal sins against him. Failing to report an accident and dishonesty.
Either way, I hope he has at least a decent BA.