All non-O visas are valid for 90 days from the day they are issued.
That means you have 90 days to enter Thailand. Once you enter Thailand, you'll receive a 90 day entry stamp. Even if you enter the last day the visa is valid, you'll receive a 90 day entry stamp.
Why didn't he get a new non-O before he traveled back to Thailand?
Does he have a Thai bank account or can get income verification from the French embassy? If either of those he can apply for a new non-O from his immigration office if they allow in-country conversions.
If you enter on a non-O visa, the first extension must be based on the same reason that you got the visa for.
You say "waiting for my retirement visa to run out" so if you actually mean that, the 90 day non-O visa based on retirement, then no, you cannot switch to marriage because the extension must be based on retirement too.
But if you actually mean that you're waiting for your 1-year extension based on retirement (an extension is not a visa), then yes, you can apply for your new extension based on a different reason as long as you meet the requirements.