Personally, I would not be the least bit offended if someone asked me to borrow a kilt for halloween. People dress up as ER doctors all the time and scrubs are not a "costume" either. Or how about people dressing up as pirates: that garb was once considered clothing...and in some parts of Ann Arbor/Boulder/Berkeley it still is worn by the trust fund hippies.

Anyway, my point is that people need to let their hair down a wee bit. If the request was asked nicely, I see no reason to be offended. So long as they were not planning on wearing those gawd awful red wigs :-)