it's the donor's responsibility to provide cost basis. in the real world say a person sells their home, as a tax return preparer we'll ask for cost basis. some times TP will provide every single document at other times just a number. if they only provide a number, we have no responsibility to verify it unless we believe that what they provided is unreasonable. . we do warn them, that if audited, they could have issues if adequate support is lacking.
