Trickle charger is a great idea for the original intent of this thread. 
(I took the thread on a bit of a detour today.). The suggestion might help other families this month, if their students decide to leave their cars home for the winter.
I used to be the only driver with two cars at home, I would alternate driving them.
You can get a trickle charger with solar panel which you put on the roof of the car. That way you don’t need a plug in power source.
Here’s one example.
https://www.amazon.com/Sunforce-50022-Battery-Trickle-Charger/dp/B0006JO0TC
It might be that the battery your D got from AAA is lower-spec than what the car requires. The spec of interest in your particular case is CCA or cold cranking amps, which as the name implies is the ability to start a car in cold weather. Check your owner’s manual or online to see what’s recommended for her car, and also look up the battery from AAA to see if it meets the minimum CCA. While you’re at it you might as well check the battery group number and amp hours as well.
For example the battery in my car is around 800 CCA and 90 aH. You can always go higher (assuming the battery fits) but never lower than manufacturer spec, although higher isn’t always necessarily better.
Don’t know the specs on our batteries, but we had a similar problem with a Jetta. Had to start it every 3 days, would be dead in 3-7 days depending on temp. (We are in Michigan). Local mechanic couldn’t find anything so took it to the dealer who couldn’t find much either, though did comment that the battery was a cheap model/brand. I asked to have the best available installed and we haven’t had a problem since. I now start it every 7 days when temps below 35 and try to drive it half an hour.