She is right. What's the matter with you?
Seriously Chris, I know absolutely nothing about pianos, but I would be easily convinced that they must suffer from disuse. Once tuned, they are taut, and closely tensioned. Time must allow them to relax, and settle, taking them out of tune. It makes perfect sense to me, plus, it's only $150, be glad she doesn't need a new piano each year.
edit: maybe ask a piano expert if it makes any sense to loosen all the strings to allow it to sit for long periods of time.