If the husband of your mother-in-law does not have either a Social Security Number or Individual Tax Identification Number, your mother-in-law can paper file her tax return as married filing separately and put NRA in the SSN box against her spouse's name.
If the husband of your mother-in-law has either a Social Security Number or Individual Tax Identification Number, then they can elect to file a married joint return treating the husband of your mother-in-law as a resident.
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Non-resident spouse
