As the chromium in alloy is "used up" (not really what happens), stainless will, indeed, begin to rust. It takes a good measure of time to happen, and that's not to say that it isn't a good thing to use, just that it isn't a panacea.

Maybe he'll wind up changing the cables every couple of years, rather than every year. It's hard to say, as it is super dependant on local conditions. A bike with stainless cables may go years without rust forming in the desert, or months next to the sea, if stored outside.