If a customer has booked on a package tour through a travel agent or tour operator, very often the travel agent or tour operator's bond with ABTA or the ATOL will pay out.
If you they have booked the elements of their holiday independently, the Scheduled Airline Failure section of the policy may provide cover. Scheduled Airline Failure covers the insolvency of the various constituent service companies involved in your trip (eg. airline, operator, hotel, car-hire company etc) where no bond applies to cover them - check the GTG policy wording for full details.