Option 1

Pull your friend aside immediately and express your concerns about their fiancé's behavior privately, explaining why you cannot support it and offering your honest perspective.

Option 2

Publicly confront the fiancé about their rude comment, or announce to other guests your disapproval of the relationship, potentially causing a scene at the party.

Option 3

Make an excuse for the fiancé's behavior to your friend and other guests, or simply avoid the situation by leaving the party early without explanation.