If this person was observant Orthodox Jewish, and you expected them to be available on Shabbat, it'd be a clear violation of religious freedom - I don't see why

If this person was observant Orthodox Jewish, and you expected them to be available on Shabbat, it'd be a clear violation of religious freedom - I don't see why a particularly religious Christian shouldn't get the same exemption. It's not reasonable to expect someone to be available 24/7 unless they were on call for this particular spell, as part of a rotation.