Buy her some new undies and suggest she save them for the off-days. It's ultimately her choice, of course.

I think the thing about no underwear with bike shorts comes from experience with chafing. I don't have any experience with chafing, because I just don't ride long enough distances, but I think it could happen. I don't wear undies under my bike shorts, though. If it doesn't happen to you, I guess you should consider yourself fortunate!

Karen