The hostess is way more out of line than your husband, OP.
I say this as a super uptight, upper middle class sort of person, who would never want to see a beer at such an event.
The role of the host is to make guests feel welcome. If there is a guest who breaks a social rule and brings unwanted food or drink for his own consumption, it's the host's duty to make reasonable accommodations. Yes, the beer added a vulgar note to an otherwise elegant event. But the accommodation was easy to make, and the host should have smiled and thought of it as an "eccentricity" on the part of one of the guests.
Because the upper classes are never crass peasants with low tastes. They're just eccentric
The fact that the hostess became upset and is now hounding you is beyond the pale, and shows how insecure she is about her social position. No REAL upper class person, or gracious host, would let this bother them.
The correct course of action is to reply by mail on your monogrammed cardstock, expressing your regret that she is upset. It's not an apology. It's an empathetic acknowledgement of her fragility.