Ok I was thinking about this more.
The problem is that any explanation of why what Gluezilla is doing is rude, relies on pointing out what is says about her and Tidy's friendship. There's no way to say "you don't value me enough for an invite but expect me to be your slave" without giving Gluezilla an opportunity to say "oh right you're hurt about the invite, this is all about your feelings/sour grapes".
Now there is one way to let her know how awful she is while making it clear you're laughing at her, not crying into your pillow - but that's by sending her a link to the threads which Tidy obviously won't want to do.
So maybe the thing to do is just one line - incredulous "Do you really not see how incredibly rude you are being?"
You could follow that up with "Although you don't value my friendship enough to invite me to your wedding after asking me to save the date, you expect me to value yours enough to give up my day to decorate your hall then leave before all your real friends arrive."
But she will just then come back with "oh I'm so sorry you are being like this" blah blah space and numbers and make it all about how she just couldn't invite you. And when she tells people about this it will be the story of how Tidy couldn't accept she wasn't invited to the wedding 