From the UK.

 

Arriving Bangkok 7th January.

 

Flying to Phnom Penh on 4th February, returning to Thailand 10th February.

 

Flying to Saigon on 6th March, returning to Thailand 12th March.

 

Flying back to the UK on 8th April.

no

Just tell him to make sure he has copies of his onward travel itineraries available.

He will probably be asked when he checks in for his initial flight to Thailand.

If he does not have a prior history of long-ish stays in Thailand, he should be OK.  But, to cover all bases on each entry to Thailand, he should have 20K Baht worth of cash to show if asked (not plastic), and proof of a flight-out within 30-days.  He will likely need those "flight out" proofs to board each flight to Thailand, in any case (is required to come without a Visa).

It is the last entry, "returning to Thailand 12th March" where he is most likely to be questioned.  Proof of his "flight home" should help with that.  Staying out longer before returning would also help - so he doesn't appear to be doing "visa runs" to stay in Thailand, - but, rather, touring the region.

 

Note that 30-day extensions of any entries may be obtained at a local immigration office.

Thank you for the replies.

 

This is simply a cost-saving exercise. My older brother and his wife wish to escape the UK winter for the benefit of their health. 

 

I have booked them a condo for 3 months but they also wish to take the opportunity to visit neighboring countries. With careful timing of these trips the 3 x 30 day visa exempt entries will cover the length of their stay. This will save the cost of 2 Tourist Visas, and 2 extensions, approx £340 in total.