The dream stone just lets him wander around into the 'dreamsphere,' so to speak, when he sleeps or meditates. He doesn't have a lot of precise control unless he's within a half a mile of the person or knows them pretty well. The further out he goes in physical distance and the less he knows the person, the less accurate he is when he wants to intentionally wander into a specific dream. Beyond that, he can just wander aimlessly and wind up wherever he winds up, with all the associated risks. It can be warded against, and player-characters can resist him seeing anything the player doesn't want seen. It's mostly just a story-driving device.