How do I deal with a colleague endangering his students?

This requires explanation. So excuse the wall of text.

I teach at a middle school. The students involved were all eighth graders. I was at lunch today when I heard shouting in a neighboring class. The teacher has no management skills at all, but this was worse than usual. After fifteen minutes or so, I single out one student who is just yelling for everyone to shut up over and over again. He really lost his cool, swearing at the other students loud enough that they actually quieted down for half a minute. When they got loud again, the same student said loudly that he was walking out of the classroom. I thought I would volunteer to escort the kid somewhere, give him a pep talk or something, but when I opened the door from my classroom to my colleague's classroom, the kid was already out the door. I asked my colleague if he wanted me to call someone, or go after him and my colleague just said 'No need.'.

Fortunately, the student went to the front office. I followed, talked with him, and volunteered to take him to my next class so that he could cool off. (It's a computer class, I figured it would distract him pretty well.)

The problem I have is that the teacher let an emotional student leave his classroom without following up on it in any way. He actively discouraged me from following up on it. I know this guy isn't the best teacher, how can I be sure that this situation does not occur again, more violently?

My current plan is to recommend the student for ED evaluation and Autism evaluation. Talking to him I got a definitely Asperger's feel. (I have a brother with Aspergers and have had several diagnosed students with Aspergers that I was pretty sure of prior to receiving their paperwork.) If I can get him on a learning plan, it will give his parents and counseling staff leverage to move his classroom. I am going to email his counselor as well, but after hearing what he went through I am totally ready to advocate for this kid.

Should I talk to the math teacher in question? Should I go directly to the principal? Would the math department head be a worthwhile compromise between the two?