Here's what I told her:
If you feel that strongly about it, you shouldn't be eating meat. Better to live a morally consistent life than to live with your head in the sand. Awareness is a good thing.
But ask yourself where you draw the line. Do you swat mosquitos? Any sympathy for ticks? Parasitic worms? Just walking outside, you may inadvertently kill any number of insects lying underfoot. If you have a functioning immune system, you're constantly killing thousands of microscopic critters just by breathing and digesting. You're a functioning part of your ecosystem; killing is unavoidable.
Even if you were to go vegan, tractors and pesticides kill wildlife, not to mention all the animals killed by trucks and the destruction of habitat caused by highways and farms.
And that's just animal life. What about the plants? With each year that passes, we discover more about the inner lives of plants. They communicate with each other and with insects. They demonstrate an awareness of their environment. They react to threats. They are sentient, if not sapient. They're as much alive as a hopping rabbit.
Even some plants kill animals for food.
So rather than trying to push the truth out of your consciousness (and into your subconscious, where it will manifest as nightmares or mental illness), embrace the truth: you are a born killer.
You have eyes on the front of your face to focus on prey instead of having them on either side of your head like prey animals do to enable them to see approaching predators from all directions. You can craft weapons to amplify your killing power. You can throw weapons...while running...in a different direction. You can make fire! You can speak to others of your species and even to dogs and horses to coordinate an attack. Walking at a normal pace for as long as you can stay awake, you can chase down any animal on four feet. (Seriously, if you just follow a deer across the open plains, never letting it rest, it will drop from exhaustion before you do.) You can even raise animals to be your food, and you won't have to chase them at all.
You are the Ultimate Predator. Get comfortable with it.