I had a former employee call my current employee that took over her job and bad mouth our agency. This occurred during business hours. She had a legitimate reason to call then went on with the other. Is it ok for me to send this ex-employee an email

I had a former employee call my current employee that took over her job and bad mouth our agency. This occurred during business hours. She had a legitimate reason to call then went on with the other. Is it ok for me to send this ex-employee an email and ask her to never do that again during business hours?