Tell her that you understand she doesn't want him fired, but that you need to protect all your other employees. He just started. It's not about her- it's about

Tell her that you understand she doesn't want him fired, but that you need to protect all your other employees. He just started. It's not about her- it's about people who won't report it. People who join your company later will be scared of existing employees, and probably won't report it. You can't be party to this.