The payment is taxable income for 2018, the year it was received. Your wife will have to pay income taxes on 2 RMDs when you file your 2018 return: the amount received Feb 2018 (the 2017 RMD) plus her payment for the 2018 RMD. People frequently have an option to delay their first RMD payment to the next tax year. If they do so, they end up with the situation you have now: you have 2 taxable payments in one year. You cannot defer the 2018 RMD to 2019.
