Family Argument: ...and one!?

[cmcramer]

Forgive my amateur terminology...here goes:

A1 begins a try for a basket.
B1 slaps his arm
Ref whistles the obvious Foul
A1 continues his customary upward shooting motion, such that if the Try was successful, it would count for an “and one” situation.
B2 Then grabs A1 arm, preventing completion of the Try.

What is proper call, if any, on B2?

Thanks for your help!