As a helo driver I'm not into flying that high, but assuming that it is a mandatory requirement to have that mask available to the pilot, you find yourself no longer able to meet the requirements and therefore you'd need to descend below 10,000 feet. This will probably make the execution of the flight as planned impossible, but it's your license on the line, not just your job.
And after landing, go find a new job..If the individual owning the thing is as irresponsible as described, what other corners will he cut that affect your safety?