Legal Question in Business Law in California

Very illegal, absolutely grounds for termination.

It violates California's wiretap laws.

It's illegal unless he told the participants he was making the recordings. Their consent would be inferred if they did not object.

Making the tapes would be grounds for termination unless the employer approved of it, or unless the salesman has a contract that limits the company's ability to fire him. That contract need not be a standard agreement signed by both sides; it could be a union contract, a commitment in an employee handbook, or even an oral promise.