Cash and credit card are good. Get cash at an ATM in Thailand.
My rule on water is only out of a bottle unless it's a really nice restaurant and only ice if I see where it came from (not broken up on the floor with a hammer).
WEAR YOUR SEATBELT! It's not that common to do although it's gotten better in the last few years. When I travel with my in-laws, I usually have to tell them to put it on. I wouldn't ride a scooter in Bangkok, not worth the risk and I'm a risk taker.
As crazy as the traffic is and lack of obeying traffic laws, I've never seen road rage.
Tell him not to fall in love with a bar girl. There are nice girls to choose from
