How well do you know him? Does he have particular hobbies, for example? If so, you could perhaps get him a gift card that covers that hobby, or buy something he could use for said hobby.
Perhaps he likes whiskey or similar? You could, perhaps, buy him something he likes to drink.
My advice, though, would be to ask your friend what her DH’s likes/dislikes are and, more importantly, if gifts are expected. (Many people are just happy to have you at the party, and don’t expect gifts, especially by a certain age, when, by then they have what they want or can get it at some point).
Whether gifts are expected or not, though, a card, I’m sure, would be appreciated.
