Hello and thank you for entrusting me to answer your question.
You are correct that California is an "at-will" employment state, meaning that unless a worker has a contract guaranteeing them employment for a specified period of time, or unless they are a government employee, they typically may be terminated at the will of the employer. (See California Labor Code Section 2922: "employment, having no specified term, may be terminated at the will of either party on notice to the other.")
As an at will employee, you are free to leave your job at any time. In exchange, your employer is free to change the requirements, promote, terminate or demote you as it sees fit and without reason. The reasons need not be fair, provided they are not otherwise discriminatory or in violation of California law or public policy.
The fact that termination occurred as a result of violence in the workplace is typically not an exception to the above rules. Therefore, an employer is typically free to terminate one employee who gets involved in a physical altercation at work while retaining the other, even if the retained employee is the one who instigated the conflict.
Your son should apply for unemployment benefits, which he may be eligible to receive although he was terminated for cause. Typically, an employee is still eligible to receive benefits unless they commit gross misconduct (stealing, showing up drunk, etc.) Getting into a fight may not qualify as gross misconduct, and assuming that your son is otherwise eligible to receive benefits, his application for them may be granted.
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