Actually, it is quite possible for dogs to get worms from eating raw meat. Roundworms, for example, could be contracted by a dog if it were to catch and eat a small prey animal (rodents, for example) that happened to have roundworms... Most worms, however, are picked up environmentally rather than through food.
I suspect what your butcher is talking about, however, is the hydatids tapeworm which can be contracted from the consumption of raw offal (especially sheep offal). Whilst not especially harmful to dogs, hydatids can be fatal to humans. Frankly though, the risk of a raw/barf fed dog contracting hydatids is extremely small. Firstly because they're generally fed on the offcuts of human-grade food (and because hydatids poses such a risk to people, a great deal of care is taken to eliminate hydatids from any animals that may become part of the human diet); and secondly because people take the simple and elementary precaution of freezing raw offal for a week before feeding it (worms, their eggs, and developing larvae are all killed by the freezing or cooking processes).
On a side note - I've fed my animals a raw diet for around 30 years and have yet to have one contract worms of any variety
