The problem is that the OP did not refuse to engage in criminal activity. The employer did not direct the OP to do anything criminal. It is not a crime under either federal or California law to simply be "in a place where illegal drugs are being used." I can sit next a guy who is doing lines coke and not commit any crime, for example.
I think that the OP's employer should have taken steps to ensure that in the future the other employee would at least not bring drugs along on work trips and for all we know the employer did that. It might have also been a good idea to have the employees in separate rooms. But while you and I might not like how the employer handled it, there was nothing illegal about what the employer did. If you think the employer did something illegal in what it requested of the OP, please share the law that the employer violated. If the employer did not violate the law, then there was no wrongful termination here. Remember, employer does not have to do the morally right thing, or the smart thing, or the popular thing. The employer needs only to ensure it doesn't violate the law. That leaves lots of room for employers to do things you may not think are right but that are perfectly legal.
Some of your posts suggest that you assume that actions you don't like are also illegal. You post here reinforces that, as you ask us to apply "common sense" to determine that the employer is acting illegally. I see plenty of potential clients who assume the same thing. But it is not a good assumption to make. The law does not always reflect common sense. Nor does the law make everything that some might see as morally wrong a crime.

