Creating a reading group with one of my students
As far as having an "unfair" advantage by preparing over the summer is concerned, I don't think that having a "club" or some other form of advanced preparation is a problem. Studying in advance is not inherently unfair if anybody can do it. (Some students take a class that a grad school might require as an undergraduate; is that unfair as well?)

The ethical question here is whether or not you will be an active instructor for that student in the coming year. Showing favoritism to individual students that would put them in an advantage—or give the appearance of having done so—should be avoided. If you're not teaching them again and anyone who wishes to participate can join, then that mitigates the problem significantly.