Who said anything about her being dangerous?
I think a better course of action would have been, force her to put the phone down, and grab her arm and force her out of the vehicle. But who's to say once the officer puts her hands on her she wouldn't try to mace him, or fight back in some other way.
She wasn't trying to explain the situation, she was giving the other person play-by-play. The conversation should have been over before the officer got to the car. But it wasn't so her asked her repeatedly to put the phone down. I agree that he pulled out the taser too quick, but once he did, he put the ball in her court. If she wants to ignore his multiple warnings, then she's gonna get shocked.
EDIT: I didn't see him trying to take the phone away, and she was on the phone giving the person exact directions where she was pulled over at. I think it could have been a dangerous situation.
She got what she deserved.