poop that <em>leaves the body</em> is the food that you eat. You digest the food in your stomach, and when that stuff is digested, you poop it out in the toilet.
Your stool passes out of your body through the rectum and anus. Another name for stool is feces. It is made of what is left after your digestive system (stomach, small intestine, and colon) absorbs nutrients and fluids from what you eat and drink. Sometimes a bowel movement isn't normal.
Tell the friend to be careful not to take too much. It does not sound like they have a negative relationship with this drug, nor serious health issues. Telling them to be cautious is the most helpful/appropriate thing to say here.
