I can't justify someone's request to not have a particular person as the SPL because she is of a particular religion. Judging here on the basis of the faith they hold is not the right criterion because that will have no effect on what services you expect from them. Just like letting someone not be a doctor because they are fat, or not hiring a driver who wakes up in the middle of the night, strap up for a while and goes back to sleep.
Ordering food home and having a problem with the delivery boy because of his faith makes no sense, just like not wanting to go to an office where there is no picture of God put up.
But if I have to guess why he might have done that, maybe ignorance, he probably thought that the driver would chant a prayer for him and package to be delivered safely and that way his food is getting effected in a way that he did not expect.
Or maybe he dislikes the practices that a general Muslim would do and thus, was not ready to accept services from them, but maybe ignore or not make an effort to know if the other services that he had been getting are from a Muslim.
But none of the guesses that I made are valid enough for him to put up a request like that.