Does
she smell or is it her clothes? I can't even fathom a musty person!
Maybe it's the type of perfume whe wears. Or maybe she wears an oil, like patchouli? It has an almost musty, earthy odor that a lot of people find unappealing.
I agree with Mr. Alura, I'm sure she's just fishing for compliments and I think subtly letting her know what a turn-off it is would be your best bet.
Lots of luck!