Hiya
I think the jury is out on milk to be honest! Its not an essential part of a cats diet, and normal cows milk can upset tums, I would guess moreso in developing kittens?
The kitten/cat milk is better, but again is usually considered supplementary to a 'normal' diet, or a treat as it is technically still a food.
It can be useful however if your cat doesn't drink much water, as its liquid of course.
I'm sure you'll get lots more feedback, but I don't think you need to worry about what your friend has said