You can cancel the vacation, up to 31 days prior and the points will go back into their account.
However, what they can do with those points, depends on what type of points were used to make the reservation.
If they are still in their banking window (first 8 months of the UY), any current UY points can still be banked into next UY.
If they borrowed any points for the reservation, those points will have to be used by the end of their UY or they will expire.
If they used banked points, then those will still have to be used by the end of their UY, as points that are banked can not be banked again.
Good luck and sorry you won't be going on the trip.