FWIW, my rule following perfectionist daughter would be bothered by being called rude, also. However, I still don't see the issue, she referred to him as a rude boy...or as by definition, an impolite boy, which he actually was guilty of being at that moment.
I know that no parent wants to hear that, myself included, but at some point we need to stop splitting hairs and own the behavior and any fall out that occurs from it.
IMO, the entire incident is as small as a blip on the radar. The boy was talking when he should have been listening, he was told that he was a rude boy for talking and that upset him.
When he came home upset I would explain to him why the teacher felt he was behaving rudely.
Keep in mind, she could have said nothing to him and marked him down on his report card (in the area that deals with respectful listening), which actually would be more aggravating, in my opinion.
She could have put it more tactfully, as well, but it sounds like she was annoyed by the interruption. At least he is aware now that this behavior is going to create friction so perhaps with parental guidance, he can come up with a plan to stay on focus and be a better listener in the future.
If it was a one time incident, I would simply ask him to apologize to the teacher on the next school day. I am sure she would be completely impressed with his thoughtful consideration and the fact that he recognized that his behavior was disruptive to her teaching.
