A few of points of context to add:
1) Making plans the night before: There had been attempts to do this, and I think Alice had been particularly keen to make firm plans, but everyone else had been quite non-committal. This was probably the same that morning as well:
Alice: What time do we want to head off?
Bob: Eh, maybe in a couple of hours.
2) Alice was just watching TV. In fact, what happened was that she was watching reruns of something, and I think the show in question finished at 11:30am. So everyone else was ready by between 11am and 11:30am, as people finished getting ready they gathered on the sofa and watched with her. We all just assumed that Alice was ready, so when the show finished, we said "shall we head off now then?". That's when Alice said that she needed to get ready.
3) Alice actually only took about 15 minutes to get ready, so she actually only cost us 15 minutes' delay.
What caused a little bit of friction was the reaction to Alice telling us that she needed to get ready. Most people in the group felt that Alice should have been getting ready at the same time as everyone else. That way, everyone would have been ready ahead of schedule and we could have got another half hour at the place we were visiting.
Alice's argument was that she was told a time-frame in which to get ready, and she met that time-frame. I think from her point of view, nobody wanted to commit to a time, and when they did it was long enough that she thought she had time to laze around for a bit (we were on holiday). From her point of view, we were changing plans with no warning.
I think if we'd known Alice was not ready (i.e. if she'd been wearing clothes that were obviously not meant for outside the house), we might have said "aren't you going to get ready?", to which she would have said "I thought we weren't leaving till 12?", and the whole thing would have been sorted.
But was it our responsibility to check whether Alice was ready rather than assuming, or was it her responsibility to tell us that she wasn't, given that it really looked like she was?