Hey, Christian! Hope I can be of help.
I personally would not go through the refinancing process for the purposes of paying off her car myself for a few reasons.
The interest rate on her Honda is astounding! 1.9% means that the car company wanted her money so much, they were financing her at a rate that barely keeps pace with inflation and that's it! If you were to pay off the car with some of the equity she has in the house, she would effectively be paying the houses new interest rate on her car (around that 4% mark) versus the great rate she already has.
Secondly, refinancing takes some upfront capital that could likely be put to better use elsewhere. My personal opinion is that anything that costs around 4-5% a year and is for a GOOD REASON (car, house, student loans, etc.) is a great value and you could take all of that money that you might be using to pay off that balance in full and invest it in the stock market - which with EXTREMELY conservative estimates can yield about 6% each year if you ride out the waves.
I am not a CPA, but the tax deductibility is always attractive to people. However, even if you could write off a portion of that tax, you will effectively be paying a higher rate overall to refinance the car into a home loan (unless the home rate it lower that 2.5%ish).
If you want anymore clarification on my personal reasoning, just let me know and I'd be happy to help. Best of luck, Christian!!!