As evidenced by Jade's post, with the proper permissions a friend can do a LOT with your private residence. If your friends are manipulating furnishings, gardening, stabling chocobos, etc, then personally I'd consider that house to be "lived in", even if the actual owner has to be away from the game for a while. Remember, the goal of auto demolition is (or should be) to free up housing that's not being used. If your friends are using your house, then yes, your house is being used.
And it's not as though this is issuing a free ticket to preserve your house forever: even if friends COULD save your house, just adding them is not enough. They'd presumably have to do exactly the same things you would do to save your house (i.e., walk through the front door every month or so).
Ideally, just having a subscription should be enough to preserve your home. If that's not going to happen, friends should be able to preserve your home, just as FC members can preserve FC homes.