First and foremost, kudos to you for taking on this challenge! It's not easy to work with vocals that are loud and harsh, and it can be even more difficult to get them to fit into a mix.
There are a few things you can do to tame these harsh frequencies. First, try using a pop filter when recording her. This will help reduce the amount of noise created by her voice. Second, use a microphone with a built-in high-pass filter. This will help reduce the amount of low-frequency noise that is being picked up by the microphone. Finally, use EQ to cut some of the harsh frequencies from her voice. You may need to experiment with different frequencies until you find the ones that work best for her vocal track.