"Backdoor" would be 401(k) to Trad IRA, then Trad converted to Roth. Your taxpayer had a retirement plan account, not an IRA, to Roth conversion. Are you sure that amount wasn't already his/her basis? If so, it's not taxable. You mention reporting basis, which is not usual within the 401(k) but if so, it already qualifies for Roth not as conversion.