After consulting with a couple of admins, it has been made clear to me that non-locketted containers, whether in a town or in the outlands (even if locked inside of a house), are actually free to loot whenever the person who happens upon them decides to do so.
That being said, they cannot take the actual containers or break anything to get to them, which means that while taking what was inside the furnaces was not against the rules, the fact that they broke the furnaces and presumably broke into the house to get TO the furnaces, that is against the rules and considered griefing.
If you could list what he broke here, I can suggest a basic fine, although I suspect it won't be much. Furnaces aren't worth a whole lot, maybe $50 or $100 maximum restitution.
