If the push was before A1 landed, then I would not award continuous motion. In other words, if the sequence was
- A1 jumps off one foot
- B1 pushes A1
- A1 lands on two feet
- A1 jumps and shoots the ball
I think the foul has occurred before the start of the shooting motion. So he's not allowed to complete the motion and score. JMO, without seeing it.
EDIT: It would be different if you thought that A1 was actually trying to shoot the ball when he jumped off one foot. If you feel that B1's foul stopped his act of shooting and forced him to land, then it's a shooting foul. But if the jump was obviously part of the jump stop, no free throws from me.