What do you do if you are called upon to interview old-fashioned bullying? One of the things about being a workplace investigator is that you should not be the judge and the jury, right? You are the investigator; theoretically, being a neutral third-party investigator is inconsistent with you being the one to make determinations about outcomes. You shouldn't decide if somebody gets to keep the job or not. You can't do both. A lot of times, it's not your call what to do about these things. I do realize it still puts you in a weird situation. Still, with something like old-fashioned bullying, you have to determine somebody being a jerk ... because it's not illegal to be a jerk, and somebody creating a hostile work environment and being a bully because of a protected class. Those things come down to when you're faced with investigating old-fashioned bullying.
Is the alleged bully an equal-opportunity bully? Does the bullying raise to the level of being a hostile work environment, or is this just an unpleasant person? Then it goes back to the client to decide what to do. Clients make decisions that sometimes it is inefficient and ineffective to have a person that is just so unpleasant in the workplace because they cost more money than they're worth by their demeanor, even though it wasn't illegal. As the investigator, it is your job to prove where that level of bullying affects that workplace.