I hope you gave your dog a good ass kicking for doing that.

I would talk to them first, apologize, A LOT, and offer to buy them another bunny.

For now though, I would be giving that dog a nice punishment for the time being.

Learn from this mistake. Don't blame this on other people as you should have checked to see if the gate was closed. Don't say, "well everyday for the past so and so and no problems" that's why it's called an accident. They do happen, and you, the owner of this dog, is your reponsibility and your fault.