Before stepping in to help or even talk to your friend, it's important to do some research. Your friend's behavior may well be related to an eating disorder, or it may not be. The best way to know if your friend has an eating disorder is to learn about it.
Eating disorders are complex illnesses. They can manifest in many ways and are often accompanied by a host of less obvious symptoms that may seriously impact your friend's physical, emotional, and mental health.
If you have an idea of which eating disorder you think your friend may be struggling with, you can begin researching that topic in particular. If not, you can read more about eating disorders in general or more common eating disorders, such as
binge eating disorder (BED),
anorexia nervosa (AN), and
bulimia nervosa (BN), to see if any of those conditions look like the disordered eating behaviors your friend is displaying.
Learning more about eating disorders can also help you better understand what your friend is going through. This can help you empathize with them, ask them more helpful questions, or better understand the type of help they may need.