I'm a slob.
I go to Mexico and eat from the pushcarts in Mexico City, and I don't get sick. I eat at roadhouses and truck stops. I drink the water if the natives do; there's a reason bottled water is so popular there. Once in a while I'll have twenty-four hours of discomfort, easily controlled with Immodium AD(TM).
My boss is a clean&neat person, like most Americans. She gets tetchy eating at the hotel restaurants. Once she had to go home early and go to the doctor.
Our friend is right for once; it's all down to intestinal flora populations. If you're a typical American, you probably have very few strains, because you keep yourself clean and sanitary. When strangers show up, war ensues. Death to the invaders! Conquest for King and country!
All bets are off when it comes to amoebae. Amoebic dysentery is a real problem.
Gratifying to realize I'm a genuine internationalist, right down in the gut :-)