how should one proceed when being bullied, harassed, and slandered by one's manager?

To be actionable (ie illegal) workplace harassment must be based on an impermissible criteria such as sex, race, age, religion, having filed a workers' compensation claim etc.  Generalized harassment because "the boss is a jerk" or "doesn't like me" is not actionble.  Neither is harassment on some more specific but not illegal ground (e.g. "I want you to quit so I can promote my friend"; "I give preferential treatment to my buddies and treat everyone else poorly", "I hate Dallas Cowboy fans").  So, in order for HR or any any lawyer or court to do anything for your fiance, he'll have to identify the reason that he his being harassed and that reason will have to be an illegal one. Unless he works for a company that has a strong and active HR Department or where the company truly values employees, it is not likely that a report to HR will do any good and it may do harm.  If he can't get transfered to another department within the company, he might consider using the rest of his time there to find a different job.