"He is convinced that he read 'somewhere' that if he put the profits into his primary residence to improve it, that it would be a write-off."

Ha. Nearly 30 years ago, there was a capital gains reinvestment provision for primary residences that was removed. I'm surprised how many people try this one, though.

Which this, apparently, is not his primary or any of his residence(s). It is either a rental, or it ended up being a flip (investment that got sold). It was never his primary residence, so let's not bring zebras to a gun fight.

Wrong on those two counts, at least.

And then teach him about improve vs expense.