What if Mr. Rivera was Jewish? Orthodox Jews also observe Saturday as their Sabbath.
I don't know any 7th Day Adventists, but they should be able to practice their religion freely. Christians are forced to work on Sundays, but everyone quit complaining about it. Nothing used to even be open on Sunday at one time. Remember "blue laws"?
It's been my experience that most Christians don't care one bit of they have to work on Sunday as long as they can at least go to church in the morning.
From the same employer, probably the same thing. I’m not aware of any requirement for an employer to honor religious holidays, though many do. The obvious problem here was his conversion, had he been an Adventist, or Orthodox Jew at the time of taking the job, he wouldn’t have been hired.
In the late 19th century the Reform Movement solved the work problem by moving the Sabbath to Sunday (they changed it back later) to coincide with the Christian day of rest.