It will be their car almost exclusively. We'll share it for things like long trips home, grocery trips, or other things we do together, but we also use my car for those things 50/50. Otherwise, it's their car to drive to work.

The savings account is a joint one. Out of the $20k we have, about $11k is hers and about $9k mine. So to afford the $15k down payment she's planning, she will need to use money from my portion of this savings account or dip into her emergency fund. Beyond that, she would also need my help to afford the loan payments on the more expensive car, dipping into my monthly contributions to savings.

One option I've given her is that I'm willing to help her pay for the car if it's under $15k total. If she decides to spend above that, I will not help with the down payment or the loan payments.