Call a vet and ask them for advice. If one of them is not feeding as well because of competition with the others there are cat milk mixes at pet stores and sometimes the grocery store. I think their mom has to help them poop by licking around that area and then the mama cat eats it to clean it up. If one of the kittens is rejected for some reason by the mama you have to rub that area with a cotton ball after feeding and clean it up after they go (but you don't have to eat it, lol) If the owner of the female cat doesn't have some kind of religious restriction against it I'd encourage her to get the cat spayed and if possible the kittens as soon as they can. There are way too many cats with no homes and in the spring and summer shelters are putting down thousands of kittens, working overtime just for euthanasia of adoptable animals. She can be spayed and still nurse her kittens if the spay is done from the side. That's what was done for Blackie and Whitenose, they were spayed when they were still nursing and their babies were able to continue to nurse.