Suppose pair production results in an electron and positron pair in an entangled system.
One person (A) measures the electron and another person (B) measures the positron. Another property of this system is the state of one particle will be the opposite of the other particle. Based on these properties person A measures the spin of the electron in the $x$ direction, using this information we can deduce the spin in the $x$ direction of the positron.
Now person B attempts to measure the spin in the $y$ direction, which will result in both of them knowing the spin in the $x$ and $y$ direction of both particles. But person B will be unable to measure the particle's $y$ spin. This is a well known scenario, and my question is how is it that person B is unable to measure the $y$-spin of the positron?
What prevents him, or how is he unable to do so? Do the states all change or does something else happen to restrict him from measuring it? Any help clearing this up would be helpful.