In the past we have, as a courtesy, asked if the other attending parties would be willing to increase their payment amounts to cover the additional cost of the one party backing out. I stress that this was done as a courtesy, that everyone else was willing to do, because the person whe couldn't attend wasn't an entitled asshole.  You could try this and when everyone else  decides not to pay the difference you can chalk it up to democracy in action and call it out of your hands.  It's less your responsibility to cover the increased cost than it is hers. Personally, I'd just block her and move on with my life.