Hi Jennifer, you could take a look at this thread and the other threads referenced there, or at the site that it is announcing http://www.melodeon.net/ which is a melodeon player specific site set up by John Spiers aka Squeezy. It contains some discussion of melodeon playing injuries plus the chance for your friend to get in touch with lots of UK melodeon players who might have advice for her...

... plus my 2p worth ... I agree with all the stuff other people have said re: posture etc. Alexander technique and/or physio might well help - worth a try anyhow? My other half (a melodeon player) has had various hand & shoulder problems exacerbated rather than caused by box playing (we think) and had acupuncture in the summer which was very successfull at relieving the pain.