I think you should tell her exactly what has happened.
I was given a save the date and subsequently told I was not invited to the wedding, fair enough, she can invite who she wants. Although a 'save the date' is normally, well, save the date because I want you at my wedding, not save the date in case I decide I am going to invite you.
She then asked me to decorate the hall so that it was all nice for the people she DID want at her wedding.
I found that insulting and I at no point agreed to do it.
If it is felt that someone is not a good enough friend to be invited to your wedding, it is very rude to expect them to decorate the venue and then bugger off, don't you think?
I am sorry that neither she nor apparently you can see that it was rude and, if I am being honest, a little hurtful.
Please do not contact me again.