My wife's male supervisor (engaged) hit on her, and asked her out many times. My wife refused him, and this supervisor stopped hitting on her, but turned everything to work harassment: far more comments for her work paper than before; hiding information to trap my wife into make mistakes; talking behind my wife's back with other supervisors (reported by another supervisor to my wife); poor performance review rating; etc. 

Because there is no material evidence, I believe it is hard to report the issue to HR right now. We are also worried that her supervisor is more "useful" for the firm so my wife will be fired if we cannot turn him down.

I'm thinking to let my wife talk to him about the issue and record the conversation. If there is any sign in the conversation, we can turn it to HR then. What do you think about it?

Except for leaving the firm, what other strategy do you suggest?

**Update #1**

I just did a research, the state we live in is "one party consent state". So it seems legal to record the conversation.

The concern right now is that whether my wife's supervisor will give her more hard time if we report the issue to HR, especially if HR talk to him. His actions, in my opinion, might seem normal by other people since they are all work related, but only my wife knows how hard it is.