I'd like to split the mortgage interest amount that can be written off between me and her. But she isn't on the title or loan. Refinancing is expensive and I'd rather just cut the amount I claim on taxes and give her the balance but am not sure if that's allowed. I don't want her to just do it only to find out she's submitted bad taxes.
No. If GF is not on the loan or deed, she is not legally obligated to pay the loan, and cannot deduct mortgage interest or property taxes paid on the home.
Furthermore, you don't want to split it with her. Mortgage interest and real estate taxes are only deductible as an itemized deduction. This means she must have enough deductions to exceed the standard deduction to get any benefit. And even then, the benefit is only incremental. It is best if you claim all the deductions and let her use the standard deduction on her return.
A taxpayer can deduct home mortgage interest if both the following conditions are met.
The taxpayer files Form 1040 and itemizes deductions on Schedule A (Form 1040).
The mortgage is a secured debt on a qualified home in which the taxpayer has an ownership interest. For more details, go to: https://www.irs.gov/publications/p936