At a guess it looks like maybe a technical issue leading to a return to the gate? In any case duty of care will apply so that means a hotel tonight near LGW if necessary.
As to compensation, your friends will have presumably been told by the flight crew why they returned to the gate? but as
mikeyfly notes probably best left for another day worrying about that.
LGW NEXT INFO WILL BE AT 1945
DELAY ZY
DELAY ZO
ESTIMATED TIME OF DEPARTURE 1830
LEFT THE GATE 1850
AIRCRAFT RETURNING TO RAMP 1854