How to politely decline?

My husband is a barber and has been cutting one of his clients for years, they aren’t friends outside of the shop but my husband obviously appreciates his clientele.

Anyway, his client invited us to his wedding and gave my husband a notice in advance that we were going to be invited, so that my husband could cut his hair the day of and such.

Well, to be honest… we don’t want to go to the wedding. I don’t know him or his fiance and my husband only knows him because he is a client. Also, I don’t want to sound pretentious but it just doesn’t even seem like something we’d enjoy, we wouldn’t know anyone there.

How do we respectfully decline?