Assuming the NRA spouse never had a nexus in AZ, the spouse would have no AZ taxable income and taxpayer may file MFS for AZ.

Where did you see that DC does not require the use of the same filing status?  My understanding from the filing instructions and DoC Code is that couples who file MFJ on federal are generally required to use either file MFJ or MFS on a combined return.

Since you have determined that it would be beneficial for your client to file jointly (probably due to the level of income the spouse had), have you verified that they would really be better off filing separate for states, given both AZ and DC allow §911?

As you may be aware, §6013 elections could trigger other reporting requirements, include FATCA and foreign trust, where applicable, and that joint accounts with non-citizen spouses could have unintended gift tax consequences.