Parma,
OP stated that this game was played under high school rules. Under high school rules, plowing into a defensive player is always regarded as interference, no matter if the defender is in the act of making a play or not. Their rule is designed to prevent contact anywhere on the field by making all avoidable contact illegal.
ASA (for one) is different. Interfering with a fielder means that a play must be prevented. On the play in question, interference could still be called if the catcher had the ball before the runnner reached the plate and there was any possibility of a throw to the pitcher. If the catcher had not yet got the ball when the collision happened...then there was no play to be made and thus no interference.
Under either rule set, if the contact was malicious or flagrant the runner should also be ejected. But just because a runner is obstructed it never gives the runner "a free shot" to purposely knock the fielder into next week!