We do not know enough about your GF's income to answer you. You have not told us if she is a W-2 employee or if she is self-employed. If she is a W-2 employee then perhaps the reason she owed is she did not have enough tax withheld from her paychecks throughout the year and she should change her W-4 at work to have more withheld from her pay to avoid owing.
https://www.irs.gov/newsroom/irs-withholding-calculator-can-help-workers-have-right-amount-of-tax-wi...
If she is an independent contractor (self-employed who gets paid in cash or receives a 1099Misc) then quite possibly she should be paying estimated quarterly taxes to avoid a penalty or a large tax bill at tax time. Post back with more information and we can help.
**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.**
