Sheep Worrying - Peter

A question for Peter. Can you give any advice regarding dogs that have a taste for sheep? It's not one of mine, its my Sister's lurcher. I had a frightening experience with him last night when he got into a sheep field. Thankfully, although Cassie followed him in, she came straight back (I followed Guy Wallace's tips when she was younger), but I seem to remember reading something about that once a dog has experienced chasing sheep it is almost impossible to train it out of them, is this true?