Free will is like temperature: a useful tool for analyzing the behavior of certain systems which are too big and complicated to model in exact detail.
If you know the positions and velocities of every atom in a box of gas, with enough compute you can predict its future to arbitrary precision; does it “have a temperature”? Irrelevant! Technically yes, I guess, but it’s sorta an epiphenomenon, screened off from reality by your exact knowledge of the initial conditions and your willingness to throw compute at your model. But if you’re less-than-perfectly omniscient, it might be more convenient to consider the box as having a “temperature” and model it more abstractly.
Substitute “person+environment”/”free will” for “box of gas”/”temperature” and that’s all still true.
(Tongue partly-but-not-entirely in cheek: if you painstakingly prepare a second box of gas with the exact same initial conditions as the first, it will have exactly the same temperature; corollary, if you painstakingly prepare a second person with the exact same initial conditions as the first, they will have exactly the same free will.)