You have not mentioned which office you will use. Some require 2 months seasoning for the NON O. Pattaya is one, as an example. Check first at the office you will use to make sure.
Someone may correct me if I am wrong, but I don't believe Chonburi Immigration allows the combination method. If this is the case, he will need to keep all of the 800k in his account.
This is very good lesson for everyone to check stamps whenever they enter Thailand. It is the first thing I do on entry. Count the days and check the date.
I have been informed a number of times not to leave Thailand during the 'Under consideration' period of a visa / extension. But that was Pattaya, so maybe other offices won't mind. As mentioned, check with your local office.
If for any reason you did not receive the visa (unlikely if you prepared properly), you can apply for a 7 day extension denied stamp to leave Thailand.
If only people realized this.... See so many comments like that. Well I never had an issue, i've been doing it for years......Everyone's time will come if they try to stay in Thailand long term as a 'Tourist'.