People behave like CF's because others enable them to get away with it.
It's all very well her parents grumbling but paying up to keep the peace whilst everyone else is left feeling (rightly) resentful.
Tbf the original quote of £50 including alcohol sounds more than fair to me and is on par with what I spent hosting this year (probably nearer to £70 per family but we had pre-dinner nibbles, lobster starter, a massive rib of beef, Xmas pudding/chocolate and raspberry tart and cheese) and the fact you only had a small amount of beef is neither here nor there given it was provided.
However it doesn't sound like you had food that was on par with what I provided, the veg was free and she reneged on the promise of alcohol.
There is no way on this earth she spent £100 per family and tbh even if she did because the beef was sprinkled in fairy dust then she choose to spend over budget and it's not fair to expect everyone else to compensate for that after the event.
In your case OP there is no way I'd pay the extra money. She's clearly happy to rip her own family off to cover other spending and it's not on.
At best I'd transfer £40 with a message saying "thanks for hosting, as per the original agreement we've popped £40 in your bank to pay for the food less alcohol". If she throws a fit then ask her to justify what she spent the money on given pudding was supplied by someone else, alcohol by the families and veg was free.