I don't think that is how the reach evolution works, it says that one of the eidolon's attacks, by which I infer it means natural attacks. If the player wants his eidolon to have reach with weapons, I'd have him pick a reach weapon.
That said, you could make your own version of the Reach evolution and have it select among weapons on the hero if you wanted. There are plenty of traits, adjustments and whatnot that pick a certain weapon with a selector for you to use as examples.
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