He has a gun pointed AT HER. And if she did shoot him, that'd be the end of her undercover work. She came there to buy stuff and she new the risk. If you can't take the heat, don't do the job since the heat is part of it.
btw
I wished she could just blow him away too. Intense scene.
What she could've tried is bluff and say she's there for her boyfriend and if the dealer wants two dead customers instead of regular money supply from them, well