Can they sue for the costs involved.

[Lotpax]

This is what Tiger's conditions say. Whether these are legally challengeable is something I am not qualified to have an opinion on.

Did they have travel insurance?

10.2 Cancellation, Changes Of Schedules:
At any time after a booking has been made we may change our schedules and/or cancel, terminate, divert, postpone, reschedule or delay any flight where we reasonably consider this to be justified by circumstances beyond our control or for reasons of safety or commercial reasons. In the event of such flight cancellation or changes to the schedule so as to depart more than four (4) hours before or after the scheduled departure time, the Passenger may elect, either:

1. To be carried at the earliest opportunity on another of our scheduled services on which space is available without additional charge and, where necessary, extend the validity of your booking; or
2. To retain the value of your fare in a credit account for your future travel provided you re-book within six (6) months there from; or
3. To receive a refund in accordance with these Terms & Conditions.

Where a cancellation or significant alteration is made to our schedule before the date of your departure, we will use reasonable efforts to inform you of any such cancellation or alteration.
10.3 Sole Remedies:
Upon the occurrence of any of the events set out in Article 10.2, the options outlined in Article 10.2 (a) to (c) are the sole and exclusive remedies available to you and we shall have no further liability to you.