Legal move. Nothing in 4-44 prohibits it. It is a textbook jump stop.
The presence of B1 is irrelevant...the proximity of another player does not affect the travel rule.
It doesn't matter "why" the player jumped. The only things that matter are:
- the fact that they caught the ball with only one foot on the floor
- the fact that they jumped off of that foot without the other foot touching
- the fact that they landed on both feet simultaneously
Having jumped off of one foot after catching the ball, no pivot has been established (defined in Articles 1 and 2). As such, Articles 3 and 4 do not apply...they are only relevant once the pivot is established.