Note that the code 7 Form 1099-R reporting the amount converted to Roth is reportable on your mother's 2018 tax return. However, only the amount in excess of her RMD is to be entered as the amount converted to Roth (although it doesn't really make any difference since the entire amount is includible in income anyway and her Roth conversion basis is no longer relevant to preparing her tax returns since her 2018 tax return is her last).
