If the letter is on company headed note paper as you say, then I assume the company are aware of the contents?
If the letter was typed by an employee of the company who was instructed to do it, then it is nothing to do with the typist they were just doing their job. But if, as I suspect, the company know nothing about it and the typist was pursuing a personal vendetta , then I would take a copy of the letter direct to the managing director of the company asking them why they are using the company name with the intent of causing offence to an innocent person.I would go direct to the company and hand the letter over in person,with the condition that it is for the personal attention of and to be only opened and read by the managing director in person.
If I was MD of a company and I discovered that an employee of mine had acted in this way , they would be dismissed for gross misconduct!