Unfortunately -- self esteem comes from self -- she's got to find it herself -- it's not something you can give to her. (though that'd be nice)
I'm still not done... I'm still thinking on this one...
Regarding volunteering, there are a lot of short term volunteering things available (my usual source is
www.volunteermatch.com - -depending on your location, there may not be as much - -but it might be a start, just even spending an afternoon painting a classroom, or gardening. YOu can't get get down on yourself when you are doing good for others.... (really)