Depends on if he gets to pick the realm he lands in, and I'm pretty sure you do get to pick the realm. If not, then yeah, just quirky luck. If he does get to pick, there's no way in hell I'm believing it's a coincidence that the happens to wind up in the worst enemy of the realm of his other character.
I don't think it warrants an OOC ban even if he did get to chose. To my knowledge there's no a rule that says you have to join a realm with good intentions, so joining a realm to cause trouble isn't cheating, like an OOC-ban would indicate. It's just not nice. And probably a little cheap.
In short: Treason, yes. Cheating, no. IC-ban him.
