Heartburn, a symptom of gastroesophageal reflux (GER; when stomach contents go back up into the esophagus), is usually caused by acid (or even bile) in the esophagus.  It is often called "heartburn" because the pain, sometimes severe, is felt in the chest, typically behind the sternum.  

Primary treatment is to try lifestyle changes, specifically starting with dietary changes to decrease chances for GER.  Dietary changes include eating smaller (more frequent) meals so the stomach is not too full and avoiding foods that may decrease the pressure of the muscle at the lower esophagus that helps prevent GER from occurring (the LES, or lower esophageal sphincter) -- including foods such as caffeine, chocolate, peppermint and alcohol.  Cigarette smoke can also increase risk for heartburn.  Position can help: stay in an upright position for a couple of hours after eating, to allow the stomach to empty, and to sleep with the head of the bed elevated to use gravity to decrease reflux from occurring.  

When these 'lifestyle changes' don't work, medicines may be necessary.  To treat the pain, an antacid such as Mylanta, Maalox, or others help to neutralize acid in the esophagus.  Calcium can cause increased acid later, so Tums and Rolaids are not recommended to treat GER.  

Other medications such as histamine receptor antagonists (H2-receptor blockers) and proton pump inhibitors (PPIs) are very effective for longer term use and should be used with physician guidance.  For your 15 year old cousin, one of these medications might be necessary.  If these fix the problem, the medication can be stopped after two months.  If the problem persists or gets better but returns when the medication is stopped, a pediatric gastroenterologist should be consulted, and an endoscopy may be done to look at the esophagus and get biopsies to make sure there is no complication of the GER and that some other problem (such as a growing problem called eosinophilic esophagitis) is not the issue.