It depends upon what country you live in and the intentions behind such a visit. It has nothing to do with her husband. Teachers like to know how former students are doing. If there is any romantic attachment on the part of your son, he should not contact her.
Is it wrong for my son to email this teacher to ask his her if it's okay for him to visit her classroom at school since she's married? Will her husband be upset and jealous about this?
Did you even bother to read the responses to your last question about this?!?
The only problem with teachers and students is the possibility of an inappropriate relationship.
If the only reason to keep in touch with the teacher is just to update them on your life and see what's up with them, then there's no reason to think there's anything inappropriate going on and no reason to be uneasy.
I would say YES... It is wrong. It puts her in a very awkward position .. Married or not. If this request is driven by infatuation .. Whether this kid is 8 or 18, it is highly inappropriate.
If this request is driven by an innocent attempt to see how a respected teacher is doing .. there is no need for the formal request via email .. one would just meet them in the hallway (and NOT during class time) and say 'Hi, how are you'. Considering there is an email request involved, it would suggest this is not such an innocent request.