It's miserable if your hands are cold and it does stop you concentrating properly. I had Oxford heated grips fitted on my first bike. Made a difference on very cold days but I didn't really like the feel of them. Too bulky. I decided not to go down the fitted mittens route because I thought they just look silly. (sorry)
Has she tried wearing a pair of silk gloves inside a normal winter pair? Also, you can now get those heat pads for inside gloves, can't you? Don't they sell them in camping shops?