OP, thanks for the update. I'm so very sad For Susan and infuriated on her behalf.
She definitely needs to talk to Dave. And for heaven's sake, it's LOUISE that should be embarrassed, not Susan!
Dave ought to just be ashamed of himself. Plain and simple. This is his SISTER. (Sheesh, even *my* brother, who takes a lot of garbage from his live-in girlfriend, risked her wrath and drama and took my side when said domestic partner decided to boycott my visit home last Christmas!)
I would still bring it up with her mother, or if she has a close, preferably older, relative. Someone who can advise her on how to approach this with Dave. Just so someone in the family knows the whole story and can quietly spread the word. There *is* a genteel way to state facts to others who might inquire about the situation.
Someone asks: Say, where's Susan? Is she ill--why is she missing her brother's wedding?
Relative/friend: Unfortunately, Louise and Dave uninvited Susan, as she was unable to wear the prosthetic and it made Louise uncomfortable.
Stating of facts *when asked* is not rude in this case, IMO. No name calling, no speculation...just the facts. If guests decide to confront Louise or no longer associate with her, well...she reaps what she sows.
As for speaking with Dave--gently telling him that she loves and supports him, and wants him to be happy, but that she has been deeply hurt are all appropriate.
I don't see this as any different, honestly, of telling a future SIL that her adopted child of a different race will disrupt the all-Caucasian line-up, so not only is the little girl not in the wedding, could she leave her kid home, or sit in the back of the church? Or telling your fiance's sister with a newly-diagnosed thyroid issue that the weight she has been putting on will make your attendant line look bad. This is discrimination. Call it for what it is. Maybe it's out of line, but I subscribe to the fact that you treat your wedding guests as people, not props.
Dave needs to seriously consider the character of the woman he is about to marry, as well as his own. Sounds like neither of them is mature enough to enter a state of matrimony.
Anyway, I'm hoping for the best for Susan. Please keep us updated. She sounds like a lovely woman and I'd happily let her wear whatever she wanted as a bridesmaid.