I'm going to take a guess and say this person might be a teenaged girl. It doesn't really matter except that no one should voluntarily not eating as a means to "diet".
Your body goes into starvation mode. It's not a healthy way to "diet". It's not teaching yourself how to handle stress in a positive way as your body and mind work together. When you starve the body, you starve your mind.
Your friend needs to learn how to eat in a healthy manner so s/he gets the proper nutrients to keep body, soul and mind functioning properly.
However, there are people out there that still maintain that going without food is ok - their body is used to not eating - their stress levels are fine - they are sleeping ok.
All these excuses would work short-term. I feel for someone that has made the decision to damage their body in this manner. Until they see that this is not the way to "diet", they won't stop, unfortunately. Hope this helps.

