I am an extremely logical person. This is exactly what I would do in his position, and I wouldn't be thinking it should cause any trouble.
The algoritam goes like this: protecting his wife from the virus is the highest priority. Can he work from home - no. Is he allowed to work at the schoolyard - well some teachers do that in sommer, so technically yes.
Well than that's it. Get a warm clothes, blanket, whatever, and teach from the schoolyard. I see no problem with that. He literally has no other option.

Whoever says he is causing problem, my question is how? If it's allowed, he can. If he can't, than none should be allowed ever. The school should make the decision, but if they are going to be the school who will ban lessons outdoors, only to force the virus into a black newborn, then they should write it down: "He was fired because he ruined our plan to give the deadly virus to his pregnant wife." I don't see a problem with that. Really. He will find a new job, especially with that recommendation.