Just use your debit card as you would here withdraw 5000 or 10000 at a time and don't flash it about. Put some away and have a little in your pocket to take out and pay for things.
In tourist places they judge prices on how you look dress and act I.e shorts and t shirt gets charged more than a pat in shirt and trousers.
Respect Thais and most will be very friendly.
Stick with his girlfriend DONT let her wander on her own a western girl is the ultimate to thai boys or men and there are bad people anywhere you go.
Loas is much nicer than Bangkok .
You don't need a visa unless staying for more than a month then you need to cross a boarder for another month visa.
Amazing place when you get away from the city.