Hello, Welcome to Just Answer and thanks for your question. Sometimes taking antibiotics for an infection will kill off friendly gut bacteria which allows Clostridium difficile (C. diff) to overgrow and cause significant diarrhea. It is more common in people who are immunocompromised before taking the antibiotic. Antacids and anti-diarrheals have nothing to do with it.
Ignore your friend's well-meaning but inaccurate advice.