Hi
We visit Thailand at least twice a year and the biggest cause for concern is tourists forgetting their common sense when they leave their home country. If they go out and get hammered and take drugs then there's a much greater chance of them having problems. It's a very religious/spiritual country and being respectful goes a long way.
Don't do anything illegal, be polite and always smile, don't get drunk and act like an arse, don't loose your cool with a Thai. Follow these rules and they'll have the best time! Although it's a shame they're only seeing the 2 largest tourist areas in the country.
As for cash, debit cards can become costly so check out fees etc from the issuing bank. The Thai ATMs charge a £3-5 fee for every withdrawal, we always take cash, which we keep in the safe, and exchange as we go, and have debit and credit cards as back up.