He might want to contact the Burmese Embassy as well. They have people who help with visa and work permit paperwork. Also there will be a record in case things escalate further.
I talked to a friend of mine in Bang Bon today. She said the police there are checking every Burmese person's paperwork because of the new penalty regulations.
I did a translation job for The Ministry of Labour recently and in it they mentioned new regulations with hefty fines for both illegal workers and those who hire them were coming, though they did not go into details. It's part of Thai Labour 4.0.
However, I personally feel that they need to rewrite the laws on what businesses need in order to be able to hire foreigners. Some companies that wish to hire foreigners legally are unable to because of the requirements.
I was recently talking to a guy who told me he's on a student visa, and studying. I asked him how it works (curious to hear his answer) and he said 1,900฿ for a 60-day extension and nearly 5,000฿ for 90 days. He said this as if it were the official policy, and he had no idea the law is 1,900฿ for an extension of up to 90 days.
I would say the acceptance letter stated in numbers 5 and 6 are essentially the same, except if perhaps the institution was affiliated with a university or other body, and thus you would need letters from both.
You will also need a work permit stating you are a volunteer which you can get from The Ministry of Labour. The Non-O visa will only allow you to be in Thailand for 90 days. At the end of that period you will have to go to Immigration with a stack of paperwork from the foundation and photos of you working with them, and most importantly with the foundation director in front of the foundation's sign, plus a letter requesting an extension, and a map showing the location of the foundation, abd 1,900฿. They will then give you a stamp for 30 days while they consider your application. At the end of 30 days you go back and get a stamp for 60 days, then repeat the process. Your passport will fill up quickly.
I was in Vientiane last week renewing my visa, dropping off the application Thursday with a group. Because of the number of people they actually opened early, 7:30 a.m. As I was not there for a B visa rather than a tourist visa (nobody I spoke to was getting an ED visa) I was put in a separate queue and got through more quickly. I was also lucky to have a fairly low queue number, 53, as opposed to others in the same group who took a little longer clearing Immigration at the border and had queue numbers around 350. So get there early.
On a side note: visa run services are no longer allowed to pick up the passports in bulk and distribute them, you must pick up your own passport, following your queue number. Because we had to wait for everyone, we didn't get back to Bangkok until 1:30 a.m.