If you want to email I think I would do it in a sort of jokey way. For example..
My husband and I were a little bemused by your letter addressing Dear Mr... etc. as we were under the impression that marriage involves two people.
We realise that the concept of marriage has changed somewhat in recent years but were not aware that a man could now be married without any other person involved. Clearly we must have missed something, unless there is a new security policy decision to make one of the two passport holders invisible.
IF so, we are somewhat concerned as invisibility could cause difficulties on presenting her passport, especially when it comes to her photograph.
Mr x and Mrs X (who was not invisible when she last checked and would rather like to be acknowledged as actually existing).