assuming company owns car.
for 2019 i would only reimburse her $ .32 per business mile. the reason being $.26 of the $ .58 standard mile rate is for depreciation. since she doesn't own it you shouldn't be reimbursing her for it. in addition, for the personal mileage you probably should be adding $ .26/ mile to her W-2. the irs requires that employees pick up as compensation such personal use. but since she's paying all cash expenses, i would only add the amount for depreciation. this amount needs to be grossed up for social security and medicare.
if you transferred title to her you have given her compensation = to what you paid for the vehicle which would need to be added to her W-2. you can't depreciate what you don't own. in addition any amount paid to her for her personal usage would also have to be added to her W-2 again gross up for social security and medicare are required.
here's some business advice. if you own the vehicle, does the insurance shes carrying cover the company? if not, tomorrow morning talk to your company's insurance agent and ask him/her if the company needs to have insurance to protect it if she's in a wreck while on company business. I would be shocked if the agent said no and the agent would probably advise you to have insurance in case she lets her insurance lapse.
