
Hi Andee,
For a comprehensive look at health problems that can beset Cairn I'd suggest you check the CTCA (Cairn Terrier Club of America) site and the Health Related Concerns area.
http://www.cairnterrier.org/health The U.S. parent club has had a health watch for many many years and has bit more aggressive about it then the UK clubs. ( I suspect it's because in the U.S. most people still get their pure bred puppies from pet stores - which of course source their supplies from puppy mills. Hence, they have had more reason to be on the offensive.) You can also check the Health Watch section of the SCTA (Southern Cairn Terrier Association) www.sctc.org.uk/health.html
The two most "common" health problems are Luxated Patellae and portal shunt. Luxated Patellae (slipping knees) may or may not be inherited, but it's common in most small pure breds and mixed breeds. Dogs with portal shunt (liver shunt) rarely survive puppyhood and those that do generally require surgery to manage. Your friend's vet can manipulate her dog's rear legs to check for luxating patella. On careful manipulation the knee will have too much movement if it's prone to slipping. It's common to many small breeds and it's not clear yet how or if it is inherited. Obviously though, a reputable breeder will not breed a dog who is known to have the problem.
Eye tests are not routinely done because the incidence of problems is low. Ocular Melanosis is under investigation. There's nothing wrong with having the eye's checked for reassurance. By the way, my regular vet happens to be a veterinary opthamologist. I asked him about eye testing for Cairns and he said that in his view the breed does not have the degree of problem where it's a requirement. (One of mine had an eye irritation, problem from sticking her head in some flowering ornamental grass in my garden .. and my vet is VERY chatty about his experiences!)