Many people have difficult relationships with food, for a variety of reasons, and the signs to look out for can be quite subtle. It would be worth spending time listening attentively to your friend to explore what is really going on. The sorts of things to look out for might be the amounts of food that she is eating, so asking what she typically eats throughout the course of the day and if there are any restrictions that she is putting on e.g. the total number of calories she eats or certain foods that she is leaving out.
Some people with eating disorders, which can occur even if people appear to have a 'normal' weight, include trying control weight through vomiting, exercising excessively, or using weight-loss medication. These things may not be obvious to you unless you ask specifically about them.
There may be difficulties going on in her life that are driving the behaviour, so it is important to find out how she is doing more generally and keep an eye on is the reason behind any such difficulties with food e.g. problems at home or school. If there is anything that seems to be off about what she is describing, do encourage her to seek help through her GP.