if it's in a closed and thick enough boardbag the isolative material of the bag will keep the cold out and keep the board fine. If he want to be sure, let him open the airvent a little. As long as it's stored dry it will be fine. I have no idea how cold -40 F is but here in Belgium and Netherlands where i keep my board temps are currently falling to 3-4 ° C at night and as long as our boards are kept in the bags when it starts to freeze they should be fine. I've kept my GO without BBag like forever like that and have seen winters with -3 ° at night and the board is still fine.
Haiko, AKA crazychemical