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I think I have solved it. The solution is inspired by Bohmian interpretation, but does not depend on it. To generalize the notion of a "friend", consider any degrees of freedom that get entangled with the spin. These degrees of freedom may constitute a macro, a meso or a micro system, my analysis will not depend on it. In particular, those degrees of freedom may be the position of the particle with spin itself. The entanglement of those degrees of freedom with the spin takes the formThat's very interesting. To make this ambiguity even more interesting, I would suggest to reformulate the Wigner friend type of paradoxes in terms of systems in which a "friend" is a mesoscopic system (say, a system made of 100 atoms) for which it is not intuitively obvious whether we should treat it as "classical" or "quantum".

$$|\psi_{\uparrow}\rangle | {\uparrow}\rangle + |\psi_{\downarrow}\rangle | {\downarrow}\rangle$$

(for simplicity I suppress the overall normalization of the state). The crucial thing to consider are wave functions in the

**position**space ##\psi_{\uparrow}(\vec{x})=\langle \vec{x}|\psi_{\uparrow}\rangle##, ##\psi_{\downarrow}(\vec{x})=\langle \vec{x}|\psi_{\downarrow}\rangle##. If those wave functions have a negligible overlap in the sense of Eq. (4) in my "Bohmian mechanics for instrumentalists", then Wigner has to treat the system as if the system effectively collapsed. Otherwise, he has not.

Why does Wigner has to treat it as if the system effectively collapsed? Because all Hamiltonian interactions are local, so the measurement made by Wigner is a local measurement. Hence he cannot measure a non-local observable. On the other hand, an observable of which the superposition above is an eigenstate is a non-local observable when the two wave functions have a negligible overlap in the position space. Hence the Wigner's measurement in this thought experiment cannot really be performed.