Dogs and adders do not go together. An adder may possibly (but probably won't) tolerate being sniffed by a dog, but a sudden movement by the dog, for example if it looks up when its person calls, may result in a strike on the nose or face.
Although the defensive bite may use less venom than a hunting bite, the effects are dramatic. The dog's head will swell up, perhaps right back to the neck and beyond. The collar will have to be loosened, or the dog may be suffocated. The dog will probably be very ill, and it may be temporarily paralysed or blinded. It may even (although rarely) die. A dog which has been bitten by an adder needs urgent treatment by a veterinary person.