Is my boss, previous co-worker and HR liable for slander of myself and my fiance?

My boss confided in a coworker as to why I resigned from my company recently. I felt bullied and harassed by my manager which I have written proof of. I also told HR that my mental health was suffering because of it. An employee not of management level sent me an email denying she made my boss turn on me but she mentioned “I hope you’re recovering from your mental health issue”. Is this slander from this employee, my manager or HR for repeating confidential information? I was treated two years ago for depression but no one within the company knows that.
This employee is now threatening to spread rumors about my fiancé who still works for this company. Everything is documented via text message.