For Head of Household filing status they have to provide over on-half the cost to maintain the home. If that is all she is contributing to the household then she cannot have HOH status and would file as Single if not legally married.
See this TurboTax support FAQ for HOH - https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-filing-status/qualify-head-household...
