[OK] What are the laws about religion in a workplace, and how to avoid religious discrimination?

I'm trying to learn more about what the laws are for a business with employees that are religious, and what has to be done to prevent religious discrimination.

To get more into specifics, this is my 14th day to work, with only one day off in between. (I had two, but they called me in because the girl who was covering me called in.) I accepted this because we are severely understaffed.

I have a coworker who is Pentecostal, and refuses to work certain days because of Church. There are only four people in our staff. And I work every night, which leaves three girls to cover first and second shift every day, rotating to let one person have a day off every now and then.

The coworker I'm asking about, we'll call her K for the question's purpose, is the only person besides me trained to do my job. So if I need a day off, she has to cover it. The only way to train the other girls is to call them in on their day off to train them.

We have all had to make sacrifices to make this schedule work. One coworker has two kids that have various things they're involved in, the other coworker has some personal problems going on and requests a day every now and then to sort it out.

But K will not sacrifice anything. She won't cover for anyone, she makes our manager change the schedule to suit her better, and has no disregard for the others. And she claims it's religious discrimination to tell her no, because all the things she rearranges the schedule for are "church related."

Is there a way around this? Is there something we can do to make her do her part in this company and stop screwing us over without actually discriminating her?

I know that sounds harsh, but we think she's using it as an excuse because she knows there's nothing we can do about it. We're all worked to the bone and she makes everything go her way with "church related" excuses.