The manual says you have to perform certain steps (or let the dealership do it) if the car stands for longer than 3 months, so I think you'll be fine.
I wouldn't let someone just idle it. If they go through the trouble of starting the car I think they should at least drive it till it gets to operating temperature.
I've let my 135i stand for 5 weeks with a connected battery and it was just like the day I left it when I came back. No, wait, it wasn't. It was even better than I remembered it
