In short, an evening event was organised about a month ago and there are six of us going. It is a pay in advance thing although not hugely expensive (less than £20). There was a minimum attendance of six people so this is important.
This morning one of the six has backed out. Another of her friends has organised something and she has decided to go to this instead. She sent a group email saying she'd decided to do the other thing instead. I think this in itself is a bit rude and that really you shouldn't back out of plans because you've had what you consider a better offer. The other things here are that one of the group is now out of pocket because she paid up front and back out friend has not offered to cover the cost and also the fact that our group is now less than the minimum number required to go. These things won't be a problem if we can find another person to take the place (but may be difficult at this short notice) but I still think it's rude. If we can't find another person, we could still go but our group may be joined with another smaller group and since they would be strangers this could be a bit awkward and one of our group does have a form of social anxiety (not sure if back out friend is fully aware of the last point).
This friend has form for similar. She backed out of a meal out with the same group last year (although restaurant booking wasn't paid for and it was easily sorted) and also delayed the booking of a trip away which almost cost us the booking entirely. I suppose I'm less inclined to be charitable in my feelings towards her doing this because of the past incidents.
AIBU?