Trust me, your friend is better off not knowing about Heartless. It was a small group of guys who wanted to know more about Darkness of the Heart that's a big part of the reason we have such a problem with Heartless back home now. Course, the fact that they weren't exactly picky about how they got that knowledge probly didn't help matters.

And pretty much the only way to deal with them is to kill them. And the only way to do that permanently is with a Keyblade, like Sora has. Otherwise, you swat them in one place, they'll just pop back up somewhere else later.