Can a Single Entry Non-ED Visa Be Extended and How Long is It Valid?

I'd have your friend contact the school for more information. I doubt schools are open, however if there is some type of extension of stay that can be acquired, the school would know about it.

>Is it the type of visa which in normal circumstances you would have been able to do a border hop and get an extra 3 months?

No. An ED Visa is a single entry visa. If your friend does a border bounce (assuming the borders were open, etc), your friend would have gotten a 1 month visa exempt, which would have been extended by an additional one month for being a tourist.

Sorry, schools are closed. No more normal circumstances. He needs to leave the country and when he can not leave, get a letter from his Embassy to request an Extension of Stay at Immigration.

Why the school not explain for him? It's all pretty standard. He can get 2 months in country extensions for 3 times.