Zeto´s sudden change of stance quite surprised Shepard. Surprised and even more convinced her, that there was determined and willful woman ready to fight for what she thought was right. She might not see her facial expressions but her stance and something in her voice, even distorted by speakers, gave her intentions away. 
She longed for justice and that was something Shepard could identify with. She still remembered her thirst for batarian blood after that slaver raid on Mindoir, or righteous rage she felt after disaster that happened to her old unit. 
Face to face such a determination, Shepard had to hold herself to not nod immediately. Could she really take her? Could she really trust her? If she would let anyone to join her little crusade Normandy would be soon crowded... 
But it was pretty much how Tali, Garrus or Wrex get on board, so she couldn´t really justify to reject the Quarian. And if she was a medical genius, as she said, she would surely be an asset to the team.
Council could have some objections, but Shepard doubted it. They cared little about how had she done her job, results were all that mattered.

"Your willingness to help is surely appreciated and I know it will sound a little bit too harsh, but I have no proof of your skills or loyalty. It would be naive of me to just let you hop in without a little test of your abilities. Like I said earlier, I have some job to do close by. It should be easy and I think it would also be a perfect test of your capabilities." Shepard was maybe little bit harsher and colder. when she spoke, but she had to set things straight first. "Also, if you join my team, you will have to accept that I am the leader. I am not very strict about rules or morale, just keep in mind that I have always the final world."