Sick Crestie - PLEASE HELP!!!

Hi all, I am posting this question in the hope that someone from this forum can help my friend with her sick crestie. She has already lost one due to this "illness" and is afraid of losing another. These are the symptoms: struggling to shed; twitching/trembling of tail and toes; go off food; eyes sunken in; toes curl up at ends. Anyone with any advice or ideas as to what could have caused this?