As we know, according to Einstein's theory of relativity, simultaneity is relative. Spacetime points that are simultaneous in one reference frame are, in general, not simultaneous in other reference frames.
Let us consider two spacetime events, S1 and S2. Physicists define S1 and S2 as "simultaneous events" if the two events occur at the same "present moment." If S1 occurs at the present moment P1, but S2 occurs at another present moment P2, physicists say that S1 and S2 are non-simultaneous events.
Let us make two assumptions:
Assumption #1: The present is a subjective, mind-dependent
Assumption #2: Simultaneity is possible if, and only if the present exists.
If we make these assumptions, we are forced to conclude that simultaneity is mind-dependent.
Here is the argument that leads to this conclusion:
If the mind does not exist, the present cannot exist. If the present does not exist, simultaneity is not possible. Therefore, if the mind does not exist, simultaneity is not possible.
In order for spacetime events to be simultaneous, the mind has to exist. Simultaneity is therefore mind-dependent.