When I was a young child, one of my daily habits was giving the leftover milk from my breakfast cereal to my cat, Patches. He enjoyed the routine and lapped up the milk with relish. He never suffered any adverse consequences from the milk he consumed.
Several years later, I decided to repeat the ritual with a different cat, Mandy. Mandy seemed to enjoy the milk as much as Patches. However, an hour later she wasn’t so happy. She was in the litter box producing explosive diarrhea. I felt very sorry for Mandy (and also a bit sorry for my father, who had to clean the litter box).
It turns out that some cats, like some humans, are lactose intolerant. Others are not. If your cat can tolerate a bit of milk every now and then without suffering adverse effects, go ahead and use it as a treat. But if the milk makes her sick, then you should avoid it.
I am going to invoke the tuna caveat here: remember that cats cannot survive on an exclusive diet of milk. Some cats have medical concerns that necessarily will exclude milk from their diet. But for healthy cats with no medical issues, small amounts of milk now and then shouldn’t cause much harm–as long as they are lactose tolerant.