My friend is traveling to Mexico. Aren't there some pills he can take so he doesn't end up with dysentary?

Pepto Bismol. Antibiotics can also be helpful but a doc may not prescribe them because most don't recommend that a healthy person take them.

Otherwise, he needs to not drink the water (or even brush his teeth in it), get all his drinks without ice, and avoid salad and fruits.

Just tell him to not drink the water (really)

Tell your friend not to drink the water, not to even brush his teeth in it. When I went to Mexico a few years ago I brought bottle water with me. Also, before I left I was put on antibiotic and had two shots, one for tetnus the other for hep a or b , I can't remember. Tell your friend to have fun and be safe!!

Nop. Tropical dysentery is caused by micro organisms that your friend might catch in food or drinks - the only way to prevent it is to take good care of hygiene. Your friends mantra should be: Wash it, peel it, cook it. Always wash your hands, bring your own anti-bacterial liquid soap. No raw food, no ice cream, no ice in drinks.

Do NOT take antibiotics preventively, as it was suggested by one poster. It can have the opposite effect since it will make your stomach more prone to infections. Discuss possible vaccinations with your family doctor.

Diarrhoea or getting sick to your stomach are what are most common from bad food or water. Dysentary is a whole other thing, more serious and rare.

Just tell your friend to avoid licking the streets and floors of public bathrooms...

There is no preventative that I know of. First timers should be careful of what and where they eat. Street food is always iffy. Bottled water and clean ice are available in most towns, look for Cristapuro signs. The best way to avoid traveler's diarrhea is to get some rest everyday, do overdo with Margaritas, eat lightly. Its a wonderful place to visit. Bien viaje