This sounds like a tough situation. What Pnance said was a very good BUT #1 can be pretty tricky in certain situations. First, the dog may not care or may not like your presence. Second, if you are going to stand firm and say no, be confident. It is true....dogs CAN sense fear. If you can't be confident DON'T do it! It is truly sad that some owners are that neglectful of their dogs.

You should definitely talk to the owner, if you know where the dog is coming from. However, some owners may be nasty or rude towards you. I would consider contacting your local police department to see about any ordinances in your area such as: leash laws, dog bite prevention, etc. Some cities and states it is a LAW that a dog MUST be on leash if not confined by a secure fence. It is not fair to your dog - especially since she may not come out of it with one cut next time.