I have a friend who lives on boiled sweet potatoes (no butter) and egg whites. is this healthy?

Probably not: Extremes of diet are never good unless there is a very specific reason to have to do this. Eating a balanced diet is the proper way to get the nutrients you need for daily living and a healthy body. By limiting a diet, you limit certain vitamins, nutriets and other essentials.

Not Completely: Dr. Profitt is correct (as usual!). Humans need a variety of foods to meet their nutritional needs, and these two alone do not provide all the body requires to function at its healthiest.