Since Hortense know's her husband is such a bore, does she attend events and socialize without him?
My parent's had a friend, Eve, who was married to a jerk. For social, property, family and a whole lot of pig headedness, Eve would not divorce her DH. I literally have no memory of Eve not being in my parents social circle and she even gave a Eulogy at my Dad's funeral. I never met her husband, even though we lived in a relatively small town. She lived her life separately, so inviting Eve without her husband was quit common and acceptable.
We had a friend in our neighborhood who was married to an alcoholic. Any event he attended ended up with a major scene. My friend did not believe in divorce and she was able to eventually get him into counseling and he's been sober for several years. But for those 5 years, friend was invited and her DH wasn't.
So in my experience there are some situations where traditional etiquette rules can be broken without any major fallout.
On the other hand, if it is a small town, most guests will be used to his antics and will ignore him. You can worn guests unfamiliar with him and recommend they stay away from him. And if your SIL has the guts get her to respond to him with "Oh, YOU must be the dirty old man OP warned me about."