Playing devil’s advocate here…
While it may be possible that there is something wrong, it may also be very possible that something is working right.
Appetite is designed to keep us at an appropriate weight for our genes. It is quite possible that due to some factor, she had gained more weight than she needed and now her appetite is slowly readjusting her weight back to a healthier level.
While a lack of appetite is one possible symptom of depression, it can also be completely normal. Without any other symptom of depression, I would not expect it to be depression at all.
In fact, most people eat more when they are depressed. It’s quite possible that your friend was depressed, and is no longer so.
Focusing on the idea that “she should be eating” can be dangerous. Studies show that people with a healthy weight use internal clues, i.e. appetite, to indicate when they should eat. Overweight people use external clues. Switching from an internal to an external method of determining when and how much to eat can very easily cause a person to become overweight, and struggle with it for the rest of their life.
I don’t know what your age is, but I’m guessing High School or College age. At this time of life, people start to guide themselves instead of being guided by their parents, and often have something happen which is quite normal, but that they have never dealt with before. They can be unwilling to ask adults about it for fear that they will be ridiculed.
I would say that, as long as your friend remains a healthy weight, is energetic, and has no other signs of depression, then there is no need to worry about her lack of appetite.