Disclaimer: This is not a comprehensive detail on feeding cats raw diets. There is much detail that I have provided links for, to continue with reading.
The best meal for a cat is a whole live rabbit, it contains all the nutrients a cat requires. The skeletal meat and bones provide a good balance of phosphorus and calcium and the organs provide essential vitamins, amino and fatty acids. So your cousin is on the right path with this diet. However there's a few issues.
Cats require regular taurine in their diet, as the body cannot store it. Taurine deficiency can lead to blindness and death in cats. Taurine is found in fresh meat. The heat of cooking destroys taurine, hence your cousins adding taurine to the cat's diet. Pet food has a synthetic taurine added to it.
Cats are lactose intolerant, they lack the enzyme lactase. Kefir, traditionally, was made with a fermentation process that reduces the lactose to lactic acid, making it a viable calcium source for cats.
There is one issue with the current day methods of making kefir. Kefir made in jars needs to be stirred to ensure that all the milk can come into contact with the kefir grains, or lactose will still be present in the mix.
Fresh raw meat
Boiling the meat destroys vitamins. If it's possible for the kitten to have fresh, uncooked chicken liver and gizzards, and fish.
It would be preferable to introduce some fresh skeletal meat into the cat's diet, to
assist in obtaining taurine requirements naturally. Even better would be to provide fresh chicken wings or chicken necks twice a week, which would also assist in meeting the calcium required for the cat within the chicken bones. (too many can cause constipation). Also as Beo suggests in the comments fish with small bones, so they get the meat and the bones, without risk of constipation. (I feed my cats this) Also never feed cats cooked bones, as they can splinter and cause blockages and/or bleeding.
The risks of feeding a cat like this is a dietary imbalance. There's more info and recipes your cousin can read up on to ensure the kitten does not miss out. The issue with dietary's deficiencies in cats, is they will not show up immediately, but are insidious and will show up over time.
I recommend reading here and having a more in depth look into it.
Easy Raw Diet Feeding for the Busy PersonFeline Nutrition Foundation
For more details on this:
Taurine in Cats By Cheryl Yuill, DVM, MSc, CVH
Taurine in Cats Dr Bruce Syme BVSc (Hons)
Kefir for Pets Food Fur Life
Feeding Your Cat Cornell University, College of Veterinarian Medicine
Don't Let Calcium/Phosphorous Ratios Scare You Feline Nutrition Foundation
What should I feed my cat? RSPCA
Skeletal meat is anything attached to the skeleton used for movement, as opposed to for example heart meat.