Do you know where he is being held? Is it still in a small place, and Vietnam right?
I've travelled in Laos alot, and also in Thailand and I know of people that have gotten out of situations with a bribe but its usually right upon the occurrence of the situation. In Thailand it is usually 10-30,000 baht for personal amount like what you have mentioned.
Id approach it by him talking to them, saying something like the shame that this would bring on his family and the shame he feels for embarrassing the country.
He needs to ask them if he can pay a fine for what he has done! That is the best way to say it.
If he is dealing with some low ranking local law they are very badly paid and he might have a chance. It is worth taking I would say, even perhaps to one of the guards rather than someone higher up. If he can get a feel for anyone that appears more approachable etc.
Lastly I would suggest that if required he do anything to stall getting moved. Once he is on his way to a big city, he is way more likley to get law that will want to make an example.
Most importantly are you able to comunicate with him and how?