Better make absolutely sure you're both ALONE when you sit down to talk this through.

And once you tell her that there IS a lock, you'd better make it clear to her that there are several people's lives hanging in the balance if you tell her any more. And see what she says to that before you tell her the rest.

Also, I've been thinking...Ron, you and me better put our heads together about how to keep our roommates from figuring out about the lock. Maybe it wouldn't be dangerous to let Dean know about it. But if Seamus figures it out, the way Parkinson did, that could really be a disaster.