My two cents, fwiw:
I think that there is some reason to be concerned if it is all just free choice reading. My DD has had quite a bit of that herself and is now--as I predicted over and over again when she was younger--unaccustomed to meeting deadlines, managing her time, working on things that she doesn't love, etc...
In school there should be SOME purpose (not busywork) behind the reading she is doing. Reading to learn about x....reading to be able to discuss....reading to compare and contrast.... At very least, perhaps an adult (teacher? mentor?) who will read some of the same books that she is reading and engage her in thoughtful, analytical discussion a couple of times per week. A caution though....if you choose to address this, be specific about what would/would not be valuable. Burying a second grader under reading assignments that are writing intensive is not positive. Motorically, writing is still a tiring a task and being able to THINK like an older student doesn't equate to being able to WRITE like an older student
