And didn't I say I do martial arts and it is much more about fighting? If done right, the agressor stops doing so as he don't need it anymore to feel confident and the victim walks upright and does not have written "victim" on their head anymore.
Second, no, one intervention is not enough, for neither kid. It takes repeated interventions as behavior goes away and comes back sometimes in different form. It is not just about walking upright either.
These are strong habits in both children when talking about repeated issues. And aggressors motivation are more complicated then just confidence (situation in family that wont go away, parents who encourage aggression etc). In martial club, you need to stop the behavior while in club (not accepted here), in home you deal with it repeatedly and need to change it in situations you don't control at all and know only half of.
The same for victim or maybe even more importantly. You have to actively teach the kid behaviors that makes it less likely bullied, in situation when the kid is not bullied right now. Even if you are naturally assertive, the kid may not be and you gotta actively work with it.