Hand himself in.
As I understand it, if Immigration collar him before he does that, it is gaol (until sorted) and a bigger fine.
Here on Samui, Immigration used to check for overstays walking onto the ferries. (People heading for border bounces.) Bigger fines and more 'bonus's'.
To be fair, I have not heard of any cases lately.
If he hands himself in - be packed and ready to go. They will possibly deport him.
Re domestic travel - I use my Thai driving licence. No passport needed. Any photo id should be OK. (I once used a membership card for Taronga Zoo by mistake.)
However, every Immigration Office interprets the laws slightly differently.
Sounds like he has a friend in the local one.