If your business has a separate policy for Harassment or Bullying follow this.
Otherwise, follow the normal grievance procedure.
You will follow the first stages of the AIMLA model:-
1) Acknowledge the complaint
2) Investigate the complaint, gather evidence - this will include speaking to the person that is the alleged perpetrator of the complaint
As the allegations against the perpetrator are potentially serious you will need to consider suspending that employee.
Remember suspension is not a punishment, you may feel the investigation or the wellbeing of either employee may be compromised by keeping them at work.
This is a potentially complex situation, so we recommend you take professional advice.