You did the right thing talking to the BO. Reputation is everything and most BO's would rather know of an issue and take care of it, than risk rumor central in their area.
I've worked at a large barn too and do know the amount of work involved and how hard it can be, BUT this kid was just being flat out lazy and not wanting to be there in the first place. IF he's asked to fill in again, just make arrangements to care for your own horse if possible those days.. CYA!
I have seen buckets hung up on a horses leg before.. not pretty. Good luck to you and I hope it all works out