How can my Swedish friend obtain a Thai retirement visa while currently in Sweden?

First of all, he doesn't qualify for a visa on arrival, so I expect you are referring to a visa-exempt entry. There is no two-entry rule, but if your friend wants a retirement visa, then he can apply in Sweden for a Non-O visa based on retirement and take it from there. He needs to have 800k baht in a Thai bank in his name only for at least two months before he can apply for the 12-month extension. If he comes to Thailand and enters visa-exempt then he must already have a Thai bank account with 800k baht before he can get a Non-o, so best to get the Non-o before he comes. It is quite a simple process, but he could go down the married extension route as you only have to have 400k in the bank.

Does he receive a state pension?

Unless he does he won't get a non-immigrant O-visa based on being over 50 years old from the Thai Embassy in Sweden.

I am not sure if it matters here, but please figure out if he has used visa-on-arrival or visa-exempt two times in the last year. Those are different things, and you might get completely wrong advice if you have that wrong. The number of days stayed last year might matter too.

Just get a non-o visa for a family visit.

He gets the 90-day Non-O in Sweden, and goes through the retirement process while in Thailand.

Best option. Apply for the non O before leaving Sweden

Why can he not write by himself/ hvorfor kan han ikke selv skrive ?..... Er du værge for ham ?

He won't be able to open a bank account on a tourist visa these days so unless he has one already he needs to get the 90 day Non-O visa before he travels. He can then open an account and transfer in the 800k baht required funds for the 12 months extension of stay