Even if you don't need one for your visa, it is highly recommended you get both a COVID and a standard health insurance. Unless you want to want to pay a 100,000THB or more hospital bill (or 700,000THB if you need surgery) if you get COVID or hit by a car.
"Probably too late to get one before December 22."
- It could be possible, or not, since it takes time for a school to prepare the needed documentation. Again, ask a school.
OR would I have to leave Thailand again just to activate that."
- You would only need to be outside of Thailand and enter Thailand to activate your visa if you would get your Non-Immigrant ED visa outside of Thailand.
Either way, that would be expensive and a waste of time. You could simply get a COVID extension and probably still convert your visa exemption to an ED visa inside Thailand (although it depends if immigration are doing that for private language schools).
I would contact a reputable school. Whether it can be done is based on that.
"But could I come in on visa exempt and transfer to Ed visa"
- It's technically possible to change a visa exemption to a Non-Immigrant ED visa inside Thailand. However, it's difficult to say. In 2020, a letter was issued by the Ministry of Education, saying they were stopping ED visas for private language schools. What came of that, or if its still in effect, you'd have to ask schools directly.
Tell your friend to have a serious talk with the school. Going outside Thailand to get a Non-Immigrant visa will be expensive and complicated.
Your friend could find a school that knows what they are doing, HOWEVER it is important to keep in mind that it takes around 3 weeks to 2 months usually for schools to prepare Non-Immigrant B visa documentation for a potential teacher.
If you have a decent school with visa staff there who are good, you should legally be able to change from a tourist visa to a Non-Immigrant B visa inside the country.
You need to cancel your current extension with the documentation provided by your school. And then apply for a COVID extension right after. Confirm with an immigration officer before doing it to make sure it's all good. You might or might not be asked for a plane ticket outside of Thailand to get approved the COVID extension. If you aren't approved, you'd be given a 7 days to leave the country.