"How does this affect the worth?

The story has already changed now by admitting to "the accident" so it's honest worth would have to be determined by someone experienced in appraising such a vehicle IMO. If it looks OK, his best bet would be to probably trade it in on another car as if the salesperson who looks it over doesn't see anything obviously wrong, I'd gather that he'd get top trade in value.
Although the honest thing to do, admitting that it's been in an accident and as a result now has a new engine will definately lower its trade in value when compared to the same car that's flawless.