so, client thinks they have a $63K expense to lower their gain and these expenses were only remembered because they are selling. I would check any depreciation schedules they might have from prior years. You might be surprised and see them on there already.

Can they tell you the year of the improvement - it might already be "fully depreciated" or close to it. You would try to take the expense but have depreciation recapture (and of course you would complete form 3115 change of accounting method to request/tell IRS about the depreciation.) so you would net back to zero and have the same capital gain. Unless it was done in the last year or two - tell them it makes no change to the net sale.