She can file jointly even if she has filed separately in the past. The IRS accepts that things change from year to year.
What is Married Filing Jointly?
Married taxpayers who choose to file a joint return will use one return to report their combined income and to deduct combined allowable expenses. Married taxpayers can select this status even if one of the spouses did not have any income or any deductions. The Married Filing Jointly status can be claimed by taxpayers who, on the last day of the tax year:
• Were married and lived together.
• Were married and living apart, but were not legally separated under a divorce or separate maintenance decree. State law governs whether taxpayers are married or legally separated under a divorce or separate maintenance decree.
• Were common law married pursuant to the laws of the state in which they live (or in the state where the common law marriage began) and the marriage has not been dissolved, such as by death or divorce.
• Are the surviving spouse who did not remarry before the end of the tax year (surviving taxpayer can file a joint return with the deceased spouse).
