what to call? could not find in rulebook!

[rc blue]

Runner on 1B.
Batter hits the ball but is thrown out at 1B.
Runner has advanced to 2B, but for an unknown reason, decided to go back to 1B after touching 2B.
Defense was not sure what to do.
Coaches told pitcher to throw the ball to 1B and tag the runner while he stood on 1B (no pitch was thrown to the next batter yet). Ump called him out.

This happened in my younger brother's baseball game, and the runner was called out. I looked for it in the ASA rule book (in case I ever had to deal with it) and could not find anything about it. I looked at rule 8, section 3.

So is a runner allowed to give up his/her base and go back to the previous one? Would it make any difference if the runner went back after the next batter was pitched to?

I am curious about other peoples' opinions and interpretations. If anyone finds a situation like this in any softball rule books, please let me know!