Daughter was told to put hair up in shower

My daughter was taken to a hairdresser by her mom and has come home saying she has to put her hair up whenever she is in the shower but not washing it. Why? What's wrong with getting it wet? And is it just that it shouldn't get fully drenched? Or any water is bad? i.e. Do I need to get her a shower cap? Or will a clip/hair tie do? The water where we live is really hard and harsh, especially on the skin. Would that add to it?