"shortly after that he sold the property"

How shortly after?

Has the incorrect basis been used on prior tax returns of the seller client? I'll answer my own  questions (Hi squirrel) since I  see  you said the gift was in 2021. Seller should report the correct basis on his 2021 return.

Are you preparing the father's 2021 gift tax return? This should also show the correct basis.

You are required to inform your client of the errors. It is up to them if they amend.

Are you sure the basis is off by $200K? Father has been shorted on depreciation expense if that is the case.  And allowed or allowable depreciation is  a basis reducing factor.

Are you the one responsible for the errors?