"Practically" in this case means "virtually," "essentially," "basically," "nearly," etc.
So, "practically nothing" means "almost nothing" or, in practice, "very little."
This much is clear. The rest of the sentence, however, is ambiguous. As previous responders have said, the sentence could refer to either a very positive or a very negative person. Personally, I agree with James and Jerry that the sentence is far more likely to mean that she finds almost no joy in anything, but technically Louis could be correct as well.