Since you mention that she is planning to work more on it, I would like to offer some suggestions.
1. The professor is a nice man and it is helpful that he mentioned learning through play etc. His concerns about unschooling may stem from not knowing unschoolers - but there may be unschoolers who would confirm his fears as well. So I would not necessarily be out to refute his arguments, but I would want to see different perspectives presented on some issues (while acknowledging that the concerns are valid).
2. For example math - right now the film seems to say that math poses a problem and "many people unschool everything but math" (or something like that). It would be nice to talk to people who dont share this view of math (sometimes inspried by fear) and who can speak to the joy of learning math in non-curricular ways. (as opposed to the ones who don't school math because "who needs math," etc, a view that one also encounters in unschoolers)
3. It would be nice to see people of color.
4. The connection btw unschooling and breastfeeding is hinted at but I think that it can come out in a more articulate way (not to say that one requires the other, but just explaining the common threads).
5. Perhaps one could allow for different views on the TV issue - not making a big issue or debate, but right now only one example is there.
Incidentally I meet the above criteria, so your sister is welcome to interview me ;-)
one suggestion from the film-making angle - the wonderful scene of the kids in the puddle ends abruptly and the next moment talks about the law, almost like one is worried about what will they think of such antics ...
maybe a little fade out at least? or other transition?