1) Self-induced vomiting after having ate a big lunch accompanied by concerns about body image despite having a "normal" body composition (the term "normal" is fairly subjective based on many factors) can be interpreted as symptoms of the condition known as bulimia nervosa. However, the diagnosis cannot be accurately determined unless the classmate is observed binge-eating uncontrollably or abnormally and then purging on an episodic basis, so more than once.
2) Resources that can provide accurate information about this eating disorder include the Mayo Clinic, Psychology Today, the Cleveland Clinic, the Anxiety and Depression Association of America (ADAA), the National Institute of Mental Health, many scholarly articles, and the Diagnostic and Statistical Manual of Mental Disorders Fifth Edition (DSM-5).
3) Unless you are knowledgeable and credentialed to deal with such conditions, you should exercise caution when it comes to dealing with this situation because the psychological aspect can be a very sensitive topic and the classmate may not want to discuss it with a peer. The actions (plural) I would suggest are:
a) have a one-to-one conversation with this classmate saying that you witnessed them vomiting on purpose and, with their previous complaints about their self-image, you are worry that they may have an eating disorder, you are concerned for their wellbeing, and you think they should seek treatment or counseling because you care about them. Then assure your classmate that you will not let any other peers know what you know, as their health concerns are private unless they choose to divulge such information. Lastly, if you know their concern is about looking "normal," let them know what was said above: "normal" is a relative term -- because what is normal to one person or group may not be normal to another -- and body appearance does not always correlate to being healthy.
b) notify the school nurse and or possibly a trusted instructor with rapport and no one else. Only people that should know are those that need to know in order to help the classmate.