How ironic, one of my co-workers told me he was going to get his 2 male cats declawed because they were "picking" on their newly accquired 10 week old shepherd/lab mix puppy. I could only shake my head at the ignorance of some people. For a dog that will grow up to be probably 90 lbs+, I think protecting the dog from the cats is the last thing they should be worried about. I gave him an article on the pain and behavioral problems that declawing can and usually does cause, and hopefully he took it to heart.
As for your friend, the best method if situation and environment control. I have 3 dogs and 3 cats, and each have different personalities and ways of interacting. But until I had observed them all with each other for for extended periods of time, they were never left alone/unsupervised with each other. I know cat scratches can happen in a blink of an eye, so I would recommend (as Fbongrl did), keeping the dog on a leash and under control. Counter-conditioning works wonderfully for this. An example would be, have the whippet on the leash, and have the cat in the room (if it is comfortable around the dog, which it sounds like it is). whenver the whippet is calm and basically ignoring the cat, its gets lots of yummy treats and praise. when it is hyper and trying to 'play' it gets ignored. Same goes for the cat. when the cat is being calm and not trying to lash out at the dog, it gets treats, a chin scratch, whatever it likes (Feline Greenies are my kitties favorite as of late).
This isn't an overnight solution, but if they take the time, I bet it will greatly reduce the number of incidents. And until the whippet can be trusted to follow the "leave it" command, the cat and the dog must be seperated or leashed (dog). no exceptions. If the cat learns that scratching is the way to get what it wants (the dog to leave it alone), this will become a learned behavior and harder and harder to break. Control and supervision is the key to a happy, multiple pet household!
Sorry for the novel, but I feel very strongly about this subject, and wish more people would realize that not all animals can just "work it out", esp animals of different species. We have to teach them how to act properly, as their guardians. And I hope they don't opt for declawing, the poor kitty (oh and softclaws are an excellent suggestion as well, as cats are much more difficult to train then dogs