[umpduck11]

Your partner was correct.Kill it,and send the runner
back. In the case of the batter doing it intentionally,
I've got interference, and with less than two out,the
runner is out.With two out, the batter is out.
In his book "The Usual Suspects",Carl Childress
addressed this very issue: "If the batter inadvertently
hinders the catcher as he is returning the ball to the
mound,he is not out unless the umpire judges the interference was intentional."
Hope this helps.