Dogs will often gravitate towards available cat food because it's richer and usually has a strong YUMMY smell to dogs. If your friend gets a decent dog food and makes that available while making the cat food unavailable the dog will start eating the dog food at some point if not immediately. Dogs are not an animal known to starve themselves to death if the food they want isn't available, only manipulating their owners into thinking they will. Most terriers are highly intelligent and adept at getting their way.
Your friend could try adding something like a little tasty canned food or even a small amount of shredded, boiled and unseasoned, skinless, boneless chicken breast, rice, organic beef or chicken baby food etc. to help jump start the process but that might also end them up in a similar situation. Even adding a few kibbles of the cat food at first might help but the dog may just pick those out. Some Nutri-Stat or Nutri-Cal (gel vitamin supplement also often used for small puppies and dogs prone to low blood sugar and working dogs) may also help make your friend feel a little better about the process. The dog will eat their dog food eventually if it's clear that playing picky will not work and that is what you eat or you don't eat period.
There are usually plenty of out of the way places to keep cat food away from the dogs in the household by placing it somewhere natural to cats but not to dogs. I am guilty of keeping my cat food on an area of the kitchen counter off to the side so the dogs aren't tempted. I also keep the litter box in the laundry room with a door that used to have a window but now is the cat portal to keep the dogs away from the litter box. They just go up and over and is their little domain. Plenty of places usually once you take a look around.
There are tons of terriers with different body types and builds and they tend to be active so the dog may be okay in that respect. Do you know what type of dog it is? A perfect or nearly perfect body weight can often be confused for 'too skinny'. I will never forget when many years ago a friend of mine saw her first Italian Greyhound at the grooming shop and burst into tears and near hysterics because the dog was starving to death and not getting enough to eat. She didn't know the breed has nearly no body fat and the dog was actually in great condition with excellent muscle tone. I showed her, explained and then brought in my two as well. Parasites and other health issues are also a possible cause for a dog being underweight or weight loss.
