I don't see a problem with it at all. This was a Christian holiday not a Muslim holiday. The fact that they came to a christmas dinner shows they are open to learn about other customs/religions. What if not only a Muslim couple came over but various different religious groups with other kinds of restrictive diets. Should she try to accomadate everyone by just serving water and bean sprouts? Assuming she made enough dishes to make a full meal that are allowable in the Muslim religion then I think she did nothing wrong.

I think the only time I would go out of my way to avoid having a certain dish as part of a dinner is if one of my guests was deathly allergic (e.g.peanuts)