I had a similar, though much less severe instance last year. Spoke to a customer who is a copper about it. The upshot was that I could have reported it, but it wasn't the owner looking after it. The person responsible at that instant would be the one prosecuted as they failed to control the dog. You've got to weigh up whether you want to drop the customer in it, and be prepared for the hassle that would probably follow. Personally, given the extent of the injury, I'd report it and drop them in it. There's no need to have a badly trained animal like that.