Player runs on field during play
 

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?

Looks like possible interference, maybe UC, not a replay.
If INT, runner closest to home.
Probable restriction or ejection depending on rule set.

Forgot to mention. USA.

What age group?

MTD, Sr.

I think this is a good point. Youngsters maybe over excited because they may have been told they were going to play their "favorite position".

Also, was this a rec league setting where there may be opportunities for learning moments?

Might it have been a Unified club?

If none of the above, and with someone who should have known better, likely an INT call, and somebody has to be called out. In this case, perhaps the runner closest to home. All (any other) runners return to last base (unless third out).

Do not know the age group. It was, indeed, rec league. From what I was told, it seemed that the young lady was excited.

I would agree with the replay call in Rec League ball depending on the level (age) of the players.

This is where the purpose of the league play does become important. Is it as a teaching the game type league, or is it a competitive league situation. Is the play 7 years old or 17 years old? In our rec league we an unwritten rule that allows some flexability for the good of the people paying to play (or have their kids play).

Upper level league play, or tourney play this would have to be called INT in my opinion.

Did she actually interfere with anything? That kinda matters too. :)

Emergency medical services, because this person's obviously flipping out!

Agree. Too hard to really rule anything without knowing what happened as/after she jumped over the ball on her way to her position.

That said, by rule, you cannot have a do-over. This is likely interference by an offensive team member not in the game, which would be an immediate dead ball and the runner closest to home is called out. Or it could be nothing if nobody had a play. There's also the possibility that R1 scored before this player affected a defensive player. Really hard to say.

Rec ball, do whatever you feel is necessary.