I haven't read all eight pages of this thread so apologies if I'm just repeating what others have said.
I think it is she who has let you down.
You were no doubt looking forward to a nice, relaxing weekend looking around Rome with your friend. It was your friend's decision to stay on after her work assignment and pay for accommodation.
Now you are expected to make your own arrangements to get to the airport and this will mean you spending more money on an overnight hotel. Then she tells you that you will go your own ways during the daytime, meeting up only in the evening, when you are expected to pay for the evening meal. She should have mentioned these issues before. I'm sure you wouldn't have minded paying for one meal and perhaps foot the bill for a few drinks during the stay but to be issued with a directive that you should pay for all the meals is, I think, very rude.
If I went abroad with a friend for just three days I would expect to spend companionable time with her looking round the city. Otherwise, what would be the point of my presence? Perhaps if the trip was for longer, some time doing your own thing would probably be nice but not three days. If she doesn't want your company then it almost looks like she's invited you so she doesn't have to pay for any meals during the weekend - presumably her week's dining will be covered by her company (and possibly even the weekend accommodation).
Alternatively, although I am not usually suspicious, like gillybob, it also sprang to my mind that she might be using you as "cover" for her staying in Rome over the weekend after her work assignment has finished.
I think her behaviour is totally out of order and if it were me I am not sure I would want to stay friends with someone who blames me for pulling out of an arrangement that she set up herself and which she then made significant changes to.