What do you mean by a “leg up”? Does she need remedial work because her skills are lacking, or is she shooting for Ivy League and wants to be top of the class- for example? Motives are unclear 
I wouldn’t make reading into a chore with any goal other than enjoyment. Efforts to make kids read seem to backfire in terms of longer term reading habits.
If her skills are good, but she doesn’t like to read, let her do something she does enjoy during the summer as long as it isn’t constant screens.
Does she have a job? What else is she doing this summer?
There are many online courses. If you make the motivation clear, we could recommend some.