What would you do if you observed a colleague who you know is HIV positive with an open sore teaching CPR to students?

You ask him in private if he is taking all necessary precautions when teaching ? Open sore ? I'm assuming on or around the mouth ? I would think it would be standard practice to wipe off the test dummy's mouth between students. Just on general principal to avoid transmitting anything. If it isn't , perhaps is the problem that should be addressed. You can't pass the virus through casual contact. Through saliva it's possible, but not likely. If you express your concerns in a reasonable manner, at the proper time, he shouldn't get upset.