It might also be worth checking with someone who teaches melodeon just in case (s)he could spot something in her teqnique that could be causing the problem.

Whatever, one does need to be careful with aches and swellings caused by playing instruments. I did know somone who tried to play mandolin/banjo through a painful elbow. Over time, he finished up creating a situation where the condition became so chronic that he can hardly play those instruments at all.