disconnect the battery and go.  there is no need to start the vehicle
while you are gone.  if you want to be really anal like me you can buy one
of those automatic maintenancing chargers that automatically turn on and
off to keep you battery up to full charge.  if you choose to do that pull
the battery and put it and the charger some where out of the way in your
friends house.  also of note here is that charging batteries give of small
amounts of toxic and explosive gas.  i've never had or heard of a problem
but it is worth mentioning.

jonathan