maybe she has a problem with what she thinks the kilt entails...."no underpants"

If I were you, I'd wear the kilt, but take a pair of pants just in case.

If you wear nice shoes, hose up, a colored button up shirt and tie you'd be ok. I'd even wear boxers just to be safe.
If she complains too much, you could just slip into a pair of Dockers or whatnot.