At the time he thought his arm was broken, thus he made the same scream that he would've made in the multiple attacker scenario.
But you said "I knew what happened to him and he was being over dramatic" which I assumed he has a habit of doing. So maybe it becomes a "cry wolf" for you. In addition, he wasn't in a fight he was training, so the energy and tension that is present in a real fight isn't present in a training session. The instructor didn't continue to strike the student as if he was in a fight against a group. So the context in which you heard the scream is not the same context of being in a fight with a group. You also knew that you weren't going to be attacked in a vicious way by your instructor. Context is everything.
breaking arms isn't that easy, your living in fantasy world, induced by to many kung fu movies
You are correct it's not easy to break an arm, However there are some techniques that are easier than others for breaking arms. Most of the easy techniques don't require that you grab the arm like in BJJ to break it. The grab is often an opportunity that may be present after breaking the arm. In the arm breaking techniques that I know. The hands are arms are position in such a way that I have the opportunity to grab. These techniques same techniques can be applied to punches, mainly jabs and linear punches heading towards the face. There are techniques for breaking the punching hand, which is easier for me to do it, only because I've trained it for so long. Breaking fingers are easy to do if you can get over the thought of breaking someone's finger. Knees are easy targets if you know the right kick to use and train it. Most people pay very little attention to their legs so it's probably easier to do that. It's the same with ankles. I'm the person is injured then you attack the injury because the injury is now the weakest point.
If you can't break arms then break hands
If you can't break hands then break fingers
If you can't break fingers then break knees
If you can't break knees then break ankles
If you can't break ankles then break toes,
If you can't break toes then grab testicles
If you can't grab testicles then bite nose, hands, neck, fingers (keep in mind some people may have diseases)
if you can't do any of those then punch and move until the opportunity comes.
If you can't punch and move then do something to cause pain and or fear.