It isn't obvious that the employer can do nothing. Most employment law requires "reasonable accommodations" of religious beliefs. I am assuming this is what Jpod is referring to when he says Title VII doesn't apply that he is saying either those rules don't apply here or this isn't a reasonable request. But it is dangerous to just dismiss this out of hand as the employer gets to decide end of story.
Let me be clear here I am NOT an expert on this part of the law so it very well maybe you either don't need to accommodate the person with ERISA plans for some reason or asking a plan to amend so it carves you out is not a reasonable accommodation. I am merely pointing out you have to ask those kinds of questions.