What would you do?

[Dan_ref]

Originally posted by Hawks Coach

...
The next question should be whether a fumble constitutes a pass. I think it does not, others must believe that it does in order to determine that the throw-in violation occurred...

Coach, you have certainly nailed this one (at least
presented it better than my long winded intuitive
argument).

Fed rules state (NCAA similar):

7-6-1 Throw-in Administration

Art 1...The throw-in starts when the ball is at the disposal
of a player of the team entitled to the throw-in. The thrower shall release the ball *on a pass* directly into the
court, except as in 7-5-7, within 5 seconds after the
throw-in starts...

7-5-7 describes the throw-in after a goal (ie players can
run the line).

4-31 Pass

A pass is movement of the ball caused by a player who
throws, bats or rolls the ball to another player.

4-21 Fumble

A fumble is the accidental loss of player control when the
ball unintentionally drops or slips from a player's grasp.

So, by rule, the call in the original play is incorrect
since the ball was fumbled by the player throwing in.
Under the rules you might argue that a 5 second violation
is possible, but I'll still contend it's best to blow the
whistle if B attempts to "steal" the fumble.