My friend who is a teacher was fired because he ?

I agree with ClergetK. If this is the reason that was given for his firing, it may be that he was teaching his own interpretation of the Constitution to the class despite having been warned not to, or he may have chosen to teach his own curriculum rather than the one he was supposed to have taught in class, to the exclusion of what he was supposed to have covered. It seems unlikely that it was just the fact of his talking about the Constitution that was responsible for his firing.

There may have been other circumstances to his removal. Teaching something that is not in the textbook is not technically wrong, but if it becomes a pattern, then the teacher is not following curricular guidelines, and that can be serious. I suspect that other things may have been going on with this teacher as well. Depending on the specific circumstance, it can be quite legitimate.

If this is a school in the US, then the Constitution is part of the 5th grade curriculum. There must have been another reason that he is not sharing with you.

This does not seem to be grounds for removal. There has to be more to this situation that you may not be aware of.

well no cause my friends teacher did the same exact thing