How can my friend transition from using a visa agent to managing his NON-O Retirement Visa extension by himself in Thailand?

I have heard of people using an agent for the retirement visa, and then later doing it themselves, without starting all over again, but I think the financial requirement, would need to be in your bank account for the previous 12 month extension, up to when the time comes to do the extension yourself, so if it hasn't, you would either need to start all over again, or pay the agent one last time to do the extension with your own finances, and then the following year you could just do it yourself, that seems the easiest way of doing it to me, it doesn't make sense starting all over again, and the year after that, having done the 65k every month for the previous 12 months, he could change over to the monthly income method, and not have to tie up the 800k in the Thai bank.

If he had an agent put money into an account for him, then he recently opened a new account with 800k thb. He will need to leave the country and start the process over again.

If it’s his original account with 800k thb, then yes, he can get a renewal for 1900 thb. Immigration has a list of the paperwork required. It’s not difficult and there are people there to help.

Depends on how anal Samut Prakan immigration is when they examine his previous extension of stay. He may not comply with the requirements as it's a "dodgy" extension. Worst case scenario, he'll have to leave - maybe go to Laos, apply for a non-O 90-day eVisa and re-enter on that

He'll need to ensure that he has met the criteria for the extension he is currently on which is unlikely if an agent provided the funds. He may need to start over again by exiting Thailand and returning on either a 90 day Non-O visa or a visa exempt entry that can be converted into a 90 day Non-O visa inside Thailand. The former is obviously less hassle and avoids the issue of having to show funds were transferred from overseas

Go to his local immigration. Get list of what they require for the 1 year extension of stay. That's a start.