Maybe that is just the way she feels comfortable cleaning them to use them again without having to launder them. It does sound a little like a germ phobia. It might also be that she just doesn't like the smell of her vaginal secretions, or the sweat at the top of her thighs, and feels like it doesn't come out well in the wash. I knew someone who used to use Listerine on the tops of her thighs to cut the sweat smell.
I can't think of any reason for this, other than maybe she is afraid of germs? But bleach would acheive the same effect while getting them clean. Why don't you just ask her? But I must stress to please ask nicely and not assume it's something 'bad', ok? - Blu