In addition, it could be important for her to review her respiration technique. Some, especially beginners, tend to only inhale from the mouth (somewhat instinctive when playing an instrument where you exhale from the mouth to produce found). It is important to also open the nose when inhaling and to inhale from the nose exclusively when there is enough time (eg. long pause, slow passages). This will give a rest to the air flow that dries the mouth - especially the inhaling (since the inhaled air is much drier than the exhaled air).