She works in a small hotel bar as bar manager.
Basically, she was bullied and harassed by the hotel owner but she doesn't want to say that outright (unless that would be best?). On numerous occasions he has screamed at her for mistakes made by her boss, verbally abused her in front of staff and customers and made misogynistic & sexist comments. Also, recently her job position was advertised as vacant to 'keep her on her toes' so this counts as scaremongering or intimidation, I guess.
So, is what I'm asking is how would I go about putting this in a formal letter of resignation without sounding blunt and malicious?
ETA: Thanks for all your help!