kimber, unless your son is being singled out for this less favorable treatment by the manager because of his race, gender, religion, national origin or some other protected characteristic, then what she is doing is not illegal. Since your son belongs to a union, I suggest he file a grievance with them and see if they're willing to do anything. Your son is also free to contact the senior store manager and see if he or she is willing to review the situation and change the decision that has been made. Walking off the job however, even if only for 10 minutes, is a big deal. It may have been the smartest thing for him to do at that moment though if he would have said or done something foolish had he not taken a few minutes to calm down.
I don't agree with Katy's advice that job abandonment is better than being fired. (a) The employer can categorize his separation any way they wish to. It's their decision whether they want to call this job abandonment. Your son has no legal venue in which to challenge what the employer wants to call this. (b) Explaining to future employers that he walked off the job is no less difficult than expaining that he was fired. At his age though, this isn't that big a deal although however he explains either to future employers, it's important he not "bash" this employer. He'll come off as a malcontent rather than someone who was treated unfairly. (c) He should apply for unemployment benefits regardless of how the employer decided to categorize the separation. For UC purposes, what the employer calls it is irrelevant. The State will decide whether your son leaving the job site for 10 minutes consituted "willful misconduct." (d) It's also possible your son isn't entirely blameless in the overall situation. She may be a lousy boss but he may not have been an ideal employee either.
A person, who is nice to you, but rude to a waiter, is not a nice person. (This is very important. Pay attention. It never fails.)