That depends on which country. The Thai embassy in some countries only allow residents of that country to apply for visas. While other Thai embassies accept applications from anyone that's physically in the country. You would want to check the embassy website to see what their policy is.
no. He has to get a new non-O visa. If he's already in Thailand then he'll need at least 14-21 days on his tourist entry and he'll need to get the list from his immigration office of requirements to convert. That will include a proof of funds requirement which is either bank balance or proof of income from the French embassy in Bangkok.