1. Experimenters have free will 2. They're made up of particles 3. Those particles therefore have free will 4. Therefore we have free will (that's why it's called a proof of free will)
Item #1 above assumes the conclusion.
Either the observer's choice of measurements of the particle is completely determined by prior events, and the observer has no free will, or the observer can at his or her whim pick a way to measure the particle. If the latter, then the particle must be able to, also at whim, pick which spin to be measured at.
He never assumes that the observer is made of particles.
How did you pull (4) out of that?