Answer
Many dogs prefer to eat cat food if given the chance because generally its higher in fat, which usually means its more appealing to the dog. If your dog is eating the cat's food in addition to her own, she will begin to become overweight and obviously your cat will lose out. Since many cat owners allow their cats to eat throughout the day this is a fairly common problem. It is highly unlikely that any direct harm will come to your dog from eating cat food, but remember that cat and dog food are specifically formulated for their species' needs. In particular, cat foods normally have higher quantities of protein, fat and fiber on a weight per weight basis and have less filling effect. Therefore, it is possible that your dog's general health could be affected if he were to eat only cat food over a long period of time.
