Judgement v Missed Rule

[Eastshire]

I was working a GJV game last night with a rookie partner. As L, I'm administering an endline throw-in. A1 throws the ball towards the sideline. A2 makes a diving bat to keep the ball inbounds (no control imo). A3, who obviously thinks she cannot go into the back court, awkwardly attempts to get the ball while remaining in the FC but ultimately goes into the BC. I do not believe A3 ever gains control before my partner whistles a BC violation.

Now this call is out of my primary (I'm only seeing it since it was near my sideline in the first place) and for all I know at the time, my partner feels A3 gained control before going into the backcourt. Do you confer with him or just go with his call?

I went with his call and talked to him about it afterward. He did indeed kick it as he didn't have A3 gaining control.