It's kinda obvious from the conversion date of 2010 that FMV at the time was the correct amount to depreciate.  But for the sale in 2020, use the actual adjusted basis and the columns F and G on Form 8949 (Code L).  

Or maybe not.  Shouldn't it go on Form 4797?  Well, not if it ceased to be a rental before 2020.  Wasn't there anything in Covid legislation that would allow those deductions to be claimed anyway?  Maybe not.  Is the interest still deductible even if it wasn't a second home?  And too bad about property taxes, if he is already maxed out at 10K.  Some of those expenses would have been deductible as miscellaneous on Schedule A, but of course we had to give those up in order to give foreign investors a tax cut.

Don't make up phantom transactions trying to trick the IRS computers. Perjury only works until it doesn't.