Try to disarm her

"Sure Tamara, whatever you say," you tell the trembling girl, giving her a reassuring smile you take your phone out of your pocket.

"Give it to me. Give it to me and I'll delete them. I don't trust you!"

"Of course," you say calmly, holding the phone out to her.

She reaches for it, the knife still ready in her other hand. You wait for the moment when her eyes leave yours and look at the phone then drop it and grab for her knife hand.