For Hopefully rational consideration:
Key question: where does the right of the individual bacome important in an on call governmental job? Everything else hinges on this. Let me break it down.
1) No one would argue that the EMT on call couldnt lead a Sunday School (or the Jewish, Islamic, Hindu, or whatever equivalent) when he was technically on call for his duties. In that situation, there is clear physical separation between place of employment (station house) and place of religious activity. Aside from location, the other key difference is CLEAR ABSENCE OF OFFICIAL ENDORSEMENT of the religious activity. However, the illustration effectively negates teh "on-call employee" aspect of the argument.
2) The location argument is two sided.
a. Would the class be a problem in his room any less than in the basement? I think you could effectively argue that a RA's "office" was his room, since that's where the majority of his student interactions would take place. Or, does being a government employee mean that he cannot have any outward hint of religion in his job -- the position that many in this thread think they are espousing. Does that meant that he cannot have a cross on his wall -- since his dorm room is also his office? Does he have to hide his Bible (or Koran, for that matter)? Can he WEAR a cross or a religious T shirt? IN other words, does the fact that he LIVES there mean that his personal practice of religion must be HIDDEN from those he responsible for serving? And, what if his religious beliefs include a command to talk about his faith (which Christianity explicitly does.) Are christians (and others with similar faith sharing beliefs) forbidden from having on cal goivernment posisitons?
b. Courts have been clear that religious organizations must be provided the same access to public facilities as secular ones. If others can hold group meetings of any sort in the dorm meeting room, religious groups must have equal access. The basement location is irrelevant; the issue is whether he indeed is precluded from teaching his fath as long as he holds the job. By this interpretaion of separation, why would it even be OK for him to attend teh thing with people who knew he was RA??
3) So, when does activity by a governent employee amount to even implicit endorsement? Do government employees need to forsake their rights to talk aboiut their faiths, as a condition of employment? Since this guy has potential resposibilities 24/7, and since Christianity teaches that faith should penetrate every part of our lives, are you seriously requiring him to quit?
OR
In the absence of coercisive behavior by the RA, wouldnt a disclaimer -- like radio stations play before controversial talk shows -- completely negate the religious establishment question? Why wouldnt this be ok? Particularly when 2 important civil rights conflict (here the right of individuals to practice their faiths vs the freedom from governement established religion), shouldnt their be openness to solutions which honor both?
Lastly, the procedural issue -- if an employer has a policy, it is completely unenforcable if it isnt written down. Period. The policy needs to have apeeals, rational, interventions all spelled out. This guy should be 100% off the hook, and any employment lawyer who wanted to could eat the university for lunch on this one.