I work with a seventh day adventist on the same shift basis, and he has no problem at all working Saruturdays. Perhaps your man is either holier than most or perhaps he is just a pain in the arse.
I too respect peoples religeous tendencies, but don't believe they should let their beliefs conflict with their work, and as such this person should find alternate employment that does not conflict, if the company is unable to accommodate his extra special needs. Having said that, any company who does go to some reasonable effort to accommodate their employees needs would be "a good employer".