Legally its a very straightforward kidnapping of a minor. There isn't any ambiguity just because she consented, the law wouldn't care. I think you were going for morally confusing, since he's doing a bad thing for a good reason, at least partially.
I kinda agree with you here. In writing that comment, I was thinking that a case of child kidnapping could be justified if it was to escape some constant, unreported domestic abuse; plus if the kid is perfectly safe or in a better state with the other party.
While iffy in a court case due to custody, obligation to report, etc., it sparks a better debate when looking at it morally because what matters is if the kid is okay.