Our warlock is level 8, and has a raven familiar. The raven scouted ahead and found some guards up on a hilltop fortress. It’s in the middle of the night, and the bows are left relatively unattended. What are the mechanics for the raven to stealthily snip the strings on the bow with its beak without being detected?

I’m looking for attack/damage/sunder/item hp/item hardness kind of answers. 

Thanks!