He can't deduct state taxes paid on a 2017 state tax return on a 2017 federal return. He would have known all the deductible state taxes when he did federal the first time. Anything paid on 2017 state return (in 2018) would go on his 2018 federal return. Oh, do you mean to amend his 2018 federal? But If he does his 2017 state return now in 2019 it will go on next year's 2019 federal return. Unless I haven't had my coffee yet today.
