Abuse Must Be Reported
It doesn't matter if you ask the person to keep what you've said a secret for fear of retaliation. They can't keep workplace abuse allegations private. They are in authority and are paid a higher salary to do the right thing and deal with sensitive issues. They don't get to pick and choose, when they're in charge.
If someone goes to an authority figure, it's a cry for help. Reporting it is step one because doing otherwise is the first part of covering up wrongdoing. You shouldn't ignore signs a person might commit suicide and you shouldn't ignore signs that someone at work is being mistreated or abused and someone else might be breaking the law.