just about everything you stated is false. Uk does not issue income affidavits. The monthly requirement is 65k not 75k. He will require 800k in a Thai bank account for his 1st yearly extension of stay and lastly moving savings are not taxed.
you don’t get to chose where you pay your taxes. You will be taxed wherever the income arises. Here the income arises in the UK so the UK will tax it. If the taxed income is then remitted to Thailand there are scenarios where it will be taxed again by Thailand.
Your UK tax liability is exactly the same, you will be taxed at source. The DTA provides for a tax credit against any Thai tax liability on remitted income.