Ah, ok — then he should have applied for a multiple-entry tourist visa before coming, and he will now have to stay in a country outside Thailand for the required time to apply for a new single-entry tourist visa.
You don't have a Non-O visa. If you applied for a 60-day tourist visa through an embassy before arriving and then got a 30-day tourist extension at your local immigration office, you're good to go unless you're married to a Thai and can apply for a 60-day extension for visiting Thai family. Your eligibility for a border run and re-entry on a 60-day visa exemption depends on your passport nation. Consider using a professional land border transport company for the trip. Officially, there are no rules for how long you need to stay out, but some border crossings make up their own rules for foreigners who do a border bounce on their own.
It is unproblematic to hire agents to assist with the application itself and to open a bank account. The problem, as I wrote, is that if you also had an agent circumvent formal rules and never actually transferred or documented the funds required for the residence permit, it will be difficult to regularize an unlawfully obtained stay on your own. The immigration officer will also understand that when you appear at immigration and present the stamps in your passport. And how would you be able to document that the required 800,000 baht have been in your account for two months prior to applying, three months after the application was approved, and never dropped below 400,000 baht during the year — when the agent’s funds were only in your account for a few seconds before being transferred back?
It is unproblematic to hire agents to assist with the application itself and to open a bank account. The problem, as I wrote, is that if you also had an agent circumvent formal rules and never actually transferred or documented the funds required for the residence permit, it will be difficult to regularize an unlawfully obtained stay on your own. The immigration officer will also understand that when you appear at immigration and present the stamps in your passport. And how would you be able to document that the required 800,000 baht have been in your account for two months prior to applying, three months after the application was approved, and never dropped below 400,000 baht during the year — when the agent’s funds were only in your account for a few seconds before being transferred back?
If you have previously used agents to handle your annual extensions, and arrangements have been made to circumvent formal residency regulations, there is a high likelihood that the immigration officer you approach will ask you to continue using an agent for further extensions. In that case, the only way to disengage from the agent system is to start over entirely with a new 90-day Non-O visa.
A tourist visa is no guarantee that you will be regarded as a genuine tourist if you have previously made multiple entries using tourist arrangements. It may slightly improve your chances in borderline cases, but if you intend to maximize visa exemptions and perform a border bounce to a nearby country, this is, after all, consistent with the nature of a multiple-entry visa.
I understand. You’ll have to decide yourself. Everything is in the discretion of the IO you’re dealing with. If you don’t have any recent entry history, it will probably be fine to cross the border and re-enter on a new visa exemption, but it is always safest to cover your stay with an appropriate visa. A METV can also give you more than eight months of stay if you make a border crossing just before the visa itself expires.
You do not receive a visa on arrival, but a 60‑day visa exemption. It is generally advisable to obtain a visa to cover your longer stays, as that is the purpose of a visa. Attempting to ‘border‑bounce’ after having maximized prior visa exemptions may result in being pulled aside for questioning and required to secure an appropriate visa, or in being denied entry.
I wasn’t entirely sure whether he had received this information directly from the embassy responsible for his area, or if he was referring to the three-month requirement in the e-visa portal. When applying based on monthly income, you must present three months of documentation. However, if you apply instead on the basis of a lump sum in your account, it should be sufficient to show that the funds are there cf "current balance", (although the requirement in the e-visa portal may appear somewhat ambiguous on this point).
If you have received this information from the embassy or the consulate responsible for your area, you must of course follow their instructions. The strictness of practice varies between different foreign missions. Others could probably have accepted just the final account balance. If you are staying in Chiang Mai, there are also several reports of a quick and smooth application process via the embassy in Vientiane—just as a tip. Best of luck in any case!