Can a professor fix a mistake in grading after the semester has ended?
Yes, the professor is responsible for mistakes she made. Not noticing a mistake doesn't excuse it. But the current situation is no longer under the control of the professor as you state the question. It is up to the administration to correct the error. 

Of course, the professor should make an appeal, along with the student, for an exception to any rule. 

So, in giving an evaluation to the professor you can validly complain about the mistake, but not about the rules made by others. For that, you need to complain elsewhere. 

In general, people are responsible for their own actions and omissions, but not those of others. But they should also work to correct past mistakes however they can. 



The lead question has been edited, so let me add:

While the professor should try to help the student fix any errors that were made, the rules might not allow it. However, I would consider systems with inflexible rules, not permitting errors to be repaired, as unethical. It saves someone a bit of work to forbid such changes, but the accuracy of the record and the fairness to students are values that far outweigh such considerations.