I am trying to help a friend make the switch from retirement visa to married visa.

Most guides say obtain a Non-O from outside the county first, then when you arrive apply for an extension.

But what if you are in Thailand already on a retirement visa? Extending the retirement visa again is not an option, due to income being just below the limit.

Just follow the requirements and if you meet them apply for an extension based on Marriage. There are many posts that cover this.

He most likely is on an "Extension of Stay" based on retirement, not a visa. Financial requirement for that, is 65,000 Baht per month income, 800,000 Baht in a Thai bank account, or a combination of income and money in the bank. Even though his income isn't enough, if he has a savings account with some money in it, he may still can qualify.

If not, the financial requirements for an "Extension of Stay" based on marriage is an income of 40,000 Baht income per month, or 400,000 Baht in a bank account, no combination on this, and the income must be his alone.

No need to get another visa, he will be extending his permission to stay. Some, immigration departments apparently don't like to change because the process is more involved for the extension based on marriage, rather than the retirement.

As said conversion would just be the reason for extension of stay. But as said above if he can up enough money into an account in his name to meet the 800k per year amount he can still use retirement (which is less paperwork for everyone).