Her filing choices are married filing jointly or married filing separately. "Single" is not a choice. If she files married filing separately she cannot get any education credits so no deduction for the student loan interest.
If you were legally married at the end of 2018 your filing choices are married filing jointly or married filing separately.
Married Filing Jointly is usually better, even if one spouse had little or no income. When you file a joint return, you and your spouse will get the married filing jointly standard deduction of $24,000 (+$1300 for each spouse 65 or older) You are eligible for more credits including education credits, earned income credit, child and dependent care credit, and a larger income limit to receive the child tax credit.
If you choose to file married filing separately, both spouses have to file the same way—either you both itemize or you both use standard deduction. Your tax rate will be higher than on a joint return. Some of the special rules for filing separately include: you cannot get earned income credit, education credits, adoption credits, or deductions for student loan interest. A higher percent of your Social Security benefits may be taxable. In many cases you will not be able to take the child and dependent care credit. The amount you can contribute to a retirement account will be affected. If you live in a community property state, you will be required to provide additional information regarding your spouse’s income. ( Community property states: AZ, CA, ID, LA, NV, NM, TX, WA, WI) If you are using online TurboTax to prepare your returns, you will need to prepare two separate returns and pay twice.
https://ttlc.intuit.com/questions/1894449-married-filing-jointly-vs-married-filing-separately
https://ttlc.intuit.com/questions/1901162-married-filing-separately-in-community-property-states
https://ttlc.intuit.com/questions/1894449-is-it-better-for-a-married-couple-to-file-jointly-or-separ...
It is not easy to compare MFJ to MFS using online TT but you can do it. Since you only get one return for each account and user ID, you have to use 3 accounts and user ID’s—one for MFJ and two for each of the MFS returns. Compare, choose, and file—and pay—accordingly.
It is much easier to do this comparison using the desktop version of TT installed from a CD or downloaded to your own computer. You pay once for the software and you can prepare multiple returns easily, and it has a “what if” feature that allows comparisons.
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
