My mom and aunt (I am not a very good knitter) have made several covers using this "knicker soakers" pattern:
http://www.borntolove.com/pattern.html#knicker%20soaker
There's no pic on the site of the finished cover, but it has a nice shorts-like shape with tight legs and they have been able to add/remove stitches to customize it quite easily...If your friend has knitting experience, it should be no problem at all