Surely she doesn't have to put it all back as it was, unless she wants it that way? It's possible to buy reproduction polystyrene roses and cornices. If it's not her house, I can't see why she's liable. Once it's all cleared away and tidied up maybe she won't be so upset - I have lived without ceilings for a while in three old houses now. Once because of the hurricane, up in the attics thankfully. I think I would ask for a couple more quotes.
Insurance - that will depend on the assessor. If it's classed as an accident she should be ok, it's a bit of a minefield.
I was at a dinner party when the ceiling fell down on our dinner. We all went hungry that night - eight guests in their finery clearing up plaster and lathes. We had just been called in by the hostess, so it was lucky we weren't all in there.
On another occasion we were just about to go to bed when the old lady next door hobbled round to see us, crying for us to rescue her budgie. We couldn't understand until we went round and couldn't see through the dust. DS was only 6 months old, but we had to get round there and clear up the worst and send her to bed. Luckily her landlord was her nephew and he dealt with it quickly - and the budgie survived, as she had put his cover on and was in the kitchen when it happened.
It's always a shock though.