I agree that Louise should have never invited Susan into the bridal party in the first place, if she felt this way. And I really don't understand Louise's motivation for uninviting Susan from the wedding entirely. Does she hope that will push Susan into staying in the wedding party and wearing the prosthetic limb? Is it supposed to be coercion? Or did she and Susan have a really heated discussion about this and so Louise just doesn't like Susan anyway and is using it as an excuse, like a "My way or the highway" just to get Susan out of her life once and for all?
I can't understand someone who thinks like Louise, but she is clearly revealing her true character before the wedding. If Dave doesn't see the red flags now, he's complicit with her meanness and his wails of "woe is me. I married someone who is awful" will fall on deaf ears in 10 years.
My question is what would you all do as guests if you heard about this situation? Even if I knew Susan to be honest, I would have to hear Louise's side. Would it be out of line to try to contact Louise and hear her side? It wouldn't be my business, as a guest. But, I'd want to know all the facts before I boycotted a wedding and cut off a friendship/relationship with Louise and Dave. I wouldn't want to attend and support their union if it were true. On the other hand, I wouldn't want to rush to judgment and boycott the wedding only to find out later that I had overreacted and that there was more to the story. I believe the OP. I'm not doubting Susan's words. I'm just saying that as a guest, I wouldn't know everything going on, and would only hear bits and pieces. How would I be firm in my decision to boycott unless I knew more.