Oh, give her the elbow, Eglantine. Your 'friend' is changing all the arrangements that she originally proposed. It is she who has let you down, not the other way around!
And apart from all the new travel arrangements you would now have to make incurring additional expenses, plus paying for her meals for three nights, she has now said that you would both 'do your own thing during the day'!!! How does that equate to a 'cheap, chill-out weekend and having a great time'? I, for one, would hate to be on my own for three days in a strange city!
And that is the real betrayal of her 'friendly' invitation.
What's she going to be doing on those three days, while you are wandering around Rome on your tod?
(And 'forgetting' her husband's birthday is a joke!)
No, you were somehow being her patsy, and you did absolutely the right thing to cancel. It's a shame that you've lost your flight money, but perhaps a cheap lesson to learn ...?
Cut your losses - and this conniving, manipulative person from your life. I'm sure there are many who deserve your friendship more!