Player runs on field during play

[fredhjr]

R1 on 3B. Batter hits ball, a player on the team at bat, with glove and mask on, runs from the home side onto the field, jumps over the batted ball and assumes the position she plays when on defense. I did not see this but was told about it. Do not know how many outs or whether there were runners on 1B & 2B. Although, IMO, that shouldn't matter when making a ruling. What was called in this instance was a replay (probably because it was so unusual). What would you call?