Help! BF arrested in Thailand for smoking weed!

No experience with it first hand (luckily).
But i think he should try to contact the embassy of his country in Thailand. They can explain to him whats going to happen next and how he might be able to avoid it.

I think you can expect that he needs to pay a huge fine (bride) or will go to prison. Penalties for drug use are quite high and prison is not a happy place in Thailand. So i would suggest to pay off any reasonable amount the police might be suggesting.

most foreigners can get off with a 'donation' after spending the night in jail. most people pay around 30,000 baht.  Otherwise, perhaps contact a law firm; they have some experience with more serious offenses.

If you are in SERIOUS trouble, you should consider the CLIFFORD CHANCE law office.
they have a VERY good reputation, but their services are certainly not a bargain. They have been involved in many high-profile cases in the country.
I always have their BKK office number 02-4018800 in my pocket.
You never know what may happen to you next in "amazing Thailand".
google for them to get more information.

(oh, had to edit the Post, just found out their number has changed)

I also suggest Chaninat and Leeds