@Chris ********************
ok, then he only has to deal with the German laws regarding the will and inheritance tax, Thai laws are as good as irrelevant.
I'm not familiar with the details of these laws. But as in so many countries a handwritten will is legal in Germany if it conforms to certain specifications : handwritten and signed by the testator. The full name, the date it is written.
Inheritance tax depends on who inherits. For his wife it's tax-free for the first half million euros. No idea if his marriage is registered in Germany, and what steps to take if not.
If I were him, I would quickly write a will to have already something in case of emergency. And then contact a notary/lawyer to get all the details sorted out.
Sure he can make one himself, but since he is dying I would make sure every angle is covered by involving a notary.
Depends also if there are other siblings who might later contest whether he was sound of mind in case he writes it himself.