John, the answer then would be for her to come over, leave her car at yours, go shopping for your food in your car, thus, she is performing a service for you and taking the car for its run at the same time. You are correct, cars do not like standing for extended periods, flat batteries, sticking/seized brakes in the short term we are talking about (1-3 months?)
Leaving my 3 month old car for 4 weeks last summer, it was a bit slow on turnover, it started ok, then the discs had a bit of stiction – you could see the rust mark from the pads on the discs for a good few weeks afterwards. Leaving it for 3 months would have been more of an issue.
In life, it's not who you know that's important, it's how your wife found out.