A friends son has just got his first Thai passport he is 11 years old, he is currently in Thailand on his UK passport he wants to travel out on his UK passport and re enter Thailand a week later on his Thai passport will this be ok as I heard that immigration can get funny about this.

Thanks

Quite acceptable !

On return keep the UK passport out of sight!

No problem, as I understand it, if he flies out and back. At land borders, they usually want to see the exit stamp from the country you just left in the same passport being used for entry. I suspect this may not be true in all cases if you have both passports to show. In particular, I cannot see how they could deny entry to a Thai citizen on a Thai passport, but you never know.

No problem by air.

IO's at land borders may try to apply alien immigration policy but his son has the right as a citizen to enter Thailand and they cannot legally deny him entry with a Thai passport.

Have a Thai friend (citizen) denied entry at Swampy on Thai passport and forced to enter on Australian passport.

Later even touched up for 20k overstay.

Keep uk passport well out of sight or even better let someone else carry it.

Change passports in the air is the way to go.

Could he go through the automatic machine at the airport instead of seeing a immigration officer ?

Have a Thai friend (citizen) denied entry at Swampy on Thai passport and forced to enter on Australian passport.

Later even touched up for 20k overstay.

Keep uk passport well out of sight or even better let someone else carry it.

Immigration been known to do that. A person just has to stand their ground to keep it from happening. They should insist on it be being done formally by doing a denial of entry stamp which would never happen.