The reason your friend doesn't think there is any problem is that she gives it to them all the time, I'm assuming. If you give them milk starting when they're kittens, and keep giving it to them, most develop the enzymes necessary to digest it okay. That doesn't mean it's good for them, or even not bad for them.
I let Zissou have a tiny lick out of the cereal bowl, but she drank some once and it gave her the squirts and now she won't drink it. Just like any other species, if you eat/drink something that makes you sick, you don't like it anymore. I still can't eat an omelete from when I was 6.