Well, if you are claiming that they will get upset stomachs, and then they don't (i.e.*** it up) then your friend is right. If it were upsetting their stomachs, then they would *** it up and then avoid it altogether. However, this doesn't mean that cow's milk is alright for them. In fact, if it is going to cause problems, then they will *** it up, but from a different end *LOL*. Here is an article that I think illustrates a fair compromise between you and your friend:
Feeding Cats Milk: The Myths - Associated Content
However, I would also stress that the jury is still out on long-term affects of cow milk consumption in both humans and cats, and that even if it doesn't make the cats sick, that doesn't mean that there will not be negative health effects on their digestive systems in the future, especially when the cats get older. Cats digestive systems are not designed to digest/accommodate lactose after weaning.