There are times to argue beliefs and times not to. A five year old can't be expected to differentiate though. Maybe just point out to her that most adults don't believe in fairies or Santa claus so she need not worry about explaining why they are wrong.

I will admit last year my ds4 (then 3) and I got a disapproving comment when he ran up shouting mummy mummy a man dressed up as Santa comed. It never occurred to me a belief in Santa extended to the santa's in shops and visiting kindy.