How would you split this cost?

[Wherehas2023gone]

Here’s the scenario
Mum and teenage son plan a 3 night trip to Europe. Mum intend her partner and also son’s friend. Apartment sleeps 6 (2 bedroom). Partner invites two relatives.

Apartment can’t be cancelled.

There was no mention of cost split when friend was invited, it has subsequently become clear a split is expected.

Partner and relatives are no longer going.

How much of the apartment cost should be attributed to the friend?

I also would expect the people who have cancelled to pay for any losses/costs they were originally responsible for and I would express that to the woman. It shouldn’t be up to you to make up their share.

You should never accept offers without knowing the exact conditions of them first. And if the exact conditions aren't clear, you should always ask for all of the conditions. When you know exactly what you're committing to, and you agree to them, that's when you actually pull the trigger and start booking flights. You've done it the other way around: You've booked the flight for your son, then you've looked into the conditions of the offer. It amazes me that adults do this all the time - becaus, assumptions. Parents assuming things instead of asking to know for sure seem to get them into trouble too often.

Hopefully you've learned your lesson. It's not easy to negotiate the terms now, after you have booked your son's flight and he's excited to go. Unless you want to argue with the other parents and risk them saying they don't want to take him after all, you're kind of a hostage to their conditions now. If they want you to pay X, I guess you've got to.

Partner and relatives who pulled out pay half, your son 10% (standard child rate?) if pushed.
Have the Mum and partner split up, hence the sudden u-turn?