I can see your friend getting sick from a salad. Not wise to eat it if you have not prepared it yourself.
The thing is though, that in many places where the salad would be a problem, it is not a native food. Take Cairo for instance; you don't eat lettuce that has been grown with Nile water (you don't even touch Nile water), its not something they grow normally, and therefore its a high risk item to be eating it in the first place. Eat the local food and you will be fine. My policy is when on vacation is you shouldn't be eating what you have at home; it could kill you. The exceptions are bananas and other fruit that can be peeled by yourself, hot boiled eggs and snickers bars if you cannot stomach what is on the menu. Someone will run down the street if they don't have any.
I lived overseas for 15 years and had all the orientations about this subject you could ever hope to hear and not hear. Also have plenty of personal experience; my family has a strong constitution and we were never ill but I sure treated plenty of people on vacation who were.
DO NOT drink bottled water unless you unscrew the cap yourself. Depending upon the restaurant, do not eat lettuce, never eat mayo, or any mayo based anything. Stay positively away from the frozen food section, only eat fresh seafood if you can personally choose it off a bed of ice and only if you can see its eyes and skin to check for freshness and if you know WHERE it was caught. I highly recommend not drinking old yak's milk; no ones constitution is that strong. It is the one thing I declined but my BIL hit me up for ciprofloxin when the chinese cure didn't work.
I never live in fear of Montezuma's revenge, the Mummy's curse or Ghengis Khan's hour glass (the sand keeps running out).