Pickoff Play (softball)

[whiskers_ump]

Have witnessed this twice this year. Neither time was I
involved, but wonder what the correct procedure is and
just how I would react.
One out runner on first, catcher throws down on attempt to
catch runner returning to first, firstbase person blocks
runner off base prior to ball reaching her, gets ball and
applies tag. Offensive coach immediately heads to first to
argue obstruction. BU appeals to PU, PU had moved inside
taking line to third base in-case of overthrow and did not
actually see entire play. Stated so to offensive coacher.
Is this strictly BU call, or should have PU observed play
before existing? This is FED ball.