Airlines used to allow you to travel with passport in one name, and tickets in another provided you carried with you your marriage cert/deed poll cert.
These days it is VERY rare for an airline to allow this, and if she tries, she may find herself stuck at the airport unable to travel.
I understand she probably wants to make a clean break as soon as possible, but the easiest and cheapest way to make sure she can still use her tickets, is to keep her old passport until she has used the the pre-booked tickets.
Just as i have continued to use my maiden name passport for a year, until i have used my maiden name tickets.
She will be very lucky to get her tickets changed without a fee, or if at all on some depending on the change fees associated with her ticket class.