jet2 have the following in their terms and conditions
If, in our reasonable opinion, you have failed to behave appropriately either in the airport or on board the aircraft or we consider you unfit to fly:
- you may be prosecuted for offences committed on board the aircraft;
- we may decide (in our reasonable discretion) to cancel your flight prior to take-off and/or, after take-off;
- divert the aircraft to offload you, in which case you must pay to us all costs and expenses which we incur of any nature whatsoever as a result of or arising out of that diversion;
- we may decide to cancel any return flight or other future flights you have with us, without refund;
- we may take any other measures we deem necessary to prevent continuation of your inappropriate conduct, including your restraint or removal from the aircraft or airport.
In all of the above circumstances, you shall not be refunded the price of your booking, and we shall not be liable for any costs you incur as a result of us refusing carriage.
You will indemnify us for all costs and expenses (including the legal costs we incur in bringing any action against you) arising from your improper conduct on board the aircraft including (but not limited to) any damage caused to the aircraft.
I would think that it is this that they will be using to issue the bill as she has agreed to the terms and conditions when purchasing the flight.
(how many pepole read the terms and conditions when making a purchase?)