A friend of mine has phases (lasting several days) in which he eats **nothing but** sugars like bags of sweets, cakes, etc. what could cause this?

Physical of Emotiona: This may be a physical or an emotional problem or a combination of both. I recommend that your friend go to his/her primary care physician and be checked out. If he/she is found to be physically sound, seek a competent mental health professional to assess the likelihood of an emotional disorder.

Diabetes: Have your patient go to a doctor for blood tests. I can see from your note that you are concerned for your friend. No it is not healthy to eat only sugar for days on end. Have a frank conversation with them and drag them to the doctor.