In our experience, it appears that unstructured free time turned out to be the absolute worst thing for my DD, we didn't realize it at the time but it led to inertia and depression. We responded by "taking the pressure off" and further reducing her responsibilities which led to further malaise. Actually, it was volunteering that finally helped bring dd out of her depressed state.
I would follow your DD's lead, if she is enjoying volunteering I would not discourage it, it is probably giving her a sense of purpose and satisfaction that is just as, if not more rewarding than hanging out with friends. In my DD's case, she actually preferred the company and conversation of the elderly volunteers with whom she was working to that of her age mates.
