When is fielder interference the proper call?

another situation, a man on first, the batter hits a high fly ball to right field. the base runner returns to the bag to tag up. the ball is not caught. the first baseman was on the bag to make a play if needed. since the ball was not caught, the runner began to run to second base and tripped up over the first baseman. he was thrown out at second base. the coaches appealed that it was fielder interference, and the umpires agreed and allowed him to remain on second base. does the fielder have to automatically vacate his position area as soon as the play is no longer involving him??? i know the base runner is entitled to the base path, but while fielding, the defensive player also has the right to the ball....where do you draw the line between these 2 rules?