Leaving a pet home alone for any extended period of time is not really recommended. Having someone go to the house to feed and water them would mean them not being alone the whole time, especially if they will stay and play awhile, so that should be okay. Leaving your ferret uncontained and alone during the times that your friend is not there to feed and water her may lead to problematic behavior. Your ferret may act out in ways that aren't going to make it easy for the person who is taking care of her while you are gone, or for you when you return home. Ferrets are naturally curious and adventurous, and without you home if she got stuck somewhere she could seriously hurt herself. I would suggest keeping her in her cage while you're away, because although it might seem she would be more comfortable to have freedom to run through the whole house, it is not safe for her to be spending that long alone in the house uncaged.