Something really weird happened with my friend tonight. We were in the Ivory King DLC, near the Garrison bonfire, right as the Hexer invades. I was a shade and he was host. I poisoned the hexer, but I died to the porcupines. My friend ran away to get the Lower Garrison bonfire, and somehow he could rest. He asked me on skype, "did you kill the invader?". I said he was poisoned, so he probably died, and asked him if he saw the "invader banished" message. He said he didn't. So whatever right. I warp to the garrison bonfire and drop again for him. While we explore and kill Maldron and get the lever and stuff, we get invaded again, by Hexer Nicolai again.
Guess he didn't die after all, lol. But I've never seen this happen before. Normally if an invader is present, you can't rest at a bonfire, and they don't just go away.
Orrrr... just occurred to me as I typed this. Could the NPC invader have "expired" his time limit?