Hello and welcome,
While the medical use of marijuana is legal under CA law, it remains illegal under federal law, and a violation of many employer drug policies, which are not required to provide an exception for employees who use it legally under state law.
The law may change at some point, but currently employees may be terminated for using the substance, even with a valid prescription, and since it is a violation of the employer's policy, they would not typically be eligible for unemployment benefits.
The only argument he may have normally is if the employer did not provide notice to him that it would be a violation of its drug policy to engage in the medical use of marijuana.
Here is a link that provides more information on the issue:
I hope you found my answer helpful, even if the law is not in your favor. I would be happy to clarify my answer if anything is not clear or you are looking for additional information. All the best to you!