Nope, she needed the element of surpise before attacking him. I have no doubt that should she have pointed a gun or knife at him and not started using it immediately he would have lunged for her to try to disarm her. The whole point to getting him in the shower was to trap him with the only way out her using a knife on him. She needed the shower as a trap and a surprise attack to disarm him... and it worked. She wouldn't have wanted to take ANY chances of his being able to over power her or cause her serious injury or even turn the weapon back on her. The element of surprise was even more important than the shower trap.
I don't think he looks surprised at all in that photo. His mouth is open, but his eyes aren't opened wide and he certainly doesn't look horrified which I'm sure he would look if he turned around and suddenly saw her there threatening him with a knife. Maybe he was saying something to her or spitting out water from the shower. Could be that's when she asked him to sit in the shower, and he thought it was a really odd request (what's sexy about a naked dude sitting in a cramped shower stall?). That last photo of his face doesn't look frightened or shocked either... he just looks tired and irritated. Frankly, it's much the same expression he had in one of his naked photos on the bed. I think after the sex he just wanted her to leave... he was expecting a conference call at 6pm and probably wanted to do some preparation for it. She probably already had brought up her wanting him to take her to Cancun the next day instead of Mimi, and he was just annoyed that this visit was once again one of her ploys to get back with him.
Besides, she only has two hands. How was she supposed to hold the camera, the gun AND the knife all at the same time and be able to use any of them with any effect?
No, she needed that element of surprise to start attacking him, and she likely dropped that camera when she first struck him with the knife (the bathroom ceiling photo).