In 1357 DR, Gareth Dragonsbane and his party somehow captured Cavil and Cavil gave precious information about the guild to Gareth. However it was discovered that was all a plot of the Grandfather to lure Gareth in a trap.
Cavil was very loyal to the Grandfather of Assassins and considered him the greatest man in north Faerûn. He never betrayed him willingly. Only torture or powerful ensorcellment could break his will.