The amount of tax due would be the same.  Even if they failed to correctly depreciate it, they still have to pay recapture based on the amount of depreciation they were supposed to have claimed.  

They do have the option to file a 3115 form with DCN 107, which would allow them to simultaneous claim the past depreciation they were supposed to have taken as a current year adjustment. 

A side note, technically depreciation recapture isn't called "capital gain", because it's not taxed at the capital gains tax rate, it's taxed at your ordinary income tax rate (but capped at a maximum of 25% for section 1250 property, meaning real estate. 

If you Google something like "depreciation recapture of unclaimed depreciation" you can find numerous articles about this.