What is the best way to go forward with this? Is there a set EU procedure she can follow to lodge the complaint and have the best chance of getting her money back?
In reverse order.
No, EU261 doesn't cover this, although if she can prove negligence/error by Transavia, this should fall under denied boarding
Do the following:
Document the correct name linked to the PNR in the reservation.
Document that the PNR on the boarding pass is identical to the PNR in the reservation
Document her true identity matching the original booking with a passport scan
Send copies of these documents by registered mail to Mattijs ten Brink, CEO and outline your demands. i.e. refund of the difference between the booked fand flown fares and compensation for denied boarding
Depending on the delay and distance, compensation ranges from €150 to €600 asper
https://europa.eu/youreurope/citizen...r/index_en.htm