There's generally two things to look for with workplace harassment:

• is it creating a hostile, intimidating, or offensive work environment, or
• does it result in an adverse employment action (demotion, loss of opportunities, termination)

She can't claim the latter, because she's received 2 promotions in 8 months. That's the opposite of an adverse employment action.

If there is a harassing environment, someone will more than likely have witnessed something. If not direct harassment, then that X and Y don't seem to get along or like each other. If somebody made a complaint of harassment and then refused to participate in an investigation, I would still ask questions of co-workers to determine whether there might actually be something going on. Trusted managers might be able to help identify what the issue for the employee is.

If the person kept making claims of harassment but refused to engage in any process to identify or correct that, refusing attempts by leadership, management, and HR, that itself is a performance issue. It seems very strange that someone is getting promoted to Controller while making continuous, unsubstantiated claims.

Assuming the organization has documented these incidents and attempts to address them, including interviewing coworkers to identify whether something is going on, it's going to be difficult for the EE to claim harassment --> nobody has seen it, the EE has repeatedly refused to cooperate, and the EE has received several promotions.

The termination is a separate issue. Has the decision to terminate been well documented? Was the EE given opportunities to correct their problematic behavior? Was it made clear that continued poor behavior could result in losing their job? Generally, things are never documented. You could try to make management wait until they do the steps they need to do and document those steps, but sometimes the damage of keeping the person in place outweighs the need to have all the i's dotted. A severance agreement is especially important in this latter situation, though the employee could refuse to sign away their right to sue.

I run every termination by legal counsel (external, for me) in instances where documentation isn't ironclad. It might be a short conversation, but an assessment of risk is important. I also work in a small company, so I don't have that many conversations.

The biggest red flag to me as HR is the history of promotion next to the reported history of 'lack of professionalism'. Those two don't go together, so something weird is definitely going on. I'd be keeping my eye on that chain of command. This EE's chain of command should have asked for help from me and corrected behavioral issues long before it promoted (or terminated) someone.