How assertive we can be sometimes depends on the situation. For instance, I've had a situations where I have had to write people up then they go straight to my boss. When they act out toward me (because I know I don't come off that assertive), they gave me a head's up to inform my boss (if I hadn't done so already). In one situation--the person got fired when she went above me because it was a temporary job where people were let go of all of the time. I have also experienced where other coworkers in a section get upset when their friend is written up. Encouragement and positive reinforcement is safer than discipline. Discipline is like war sometimes--there can be unintended/unforeseen negative consequences. Your coworker will probably settle down or escalate.