You hear the litter tray rattle and find your dog with litter on their muzzle. Access is the first thing to change. Move the box to a place the cat can reach but the dog cannot, using a secure gate or cat-sized opening if it is safe for your household. The cat must still have easy, private access, a setup that makes the cat avoid the tray creates another problem.
If your dog has swallowed a substantial amount of litter, seems unwell, vomits, has abdominal pain, or strains to pass stool, call your veterinarian promptly. Cat feces can carry organisms, and litter can cause digestive trouble. A dog who repeatedly eats non-food material should be assessed by a veterinarian, not only redirected with a cue.
Keep the reward out of reach
Scoop the box frequently and use a bin the dog cannot open. Supervise the dog around the area until the new barrier is reliable. Give appropriate food puzzles or sniff games elsewhere. Teach "leave it" with harmless objects away from the cat's tray and reward turning toward you, but do not depend on the cue when nobody is watching.
Avoid scolding the dog after finding evidence later, they cannot link a delayed reprimand to the earlier visit. Think of this as a room-layout problem first and a training opportunity second. A protected box is better for both pets.
Sources: PetMD: Why do dogs eat cat poop?. VCA Animal Hospitals: Pica in Dogs.