Go on the Defense
Because your coworker is not willing to speak up you cannot go on the offense, as such you need to go on the defense. There are three general categories/goals behind going on the defense in this situation:
- Minimize opportunities
- Maximize chance of getting caught
- Empower employees
I hope that just by mentioning those categories that you already are thinking of ways and things that can be done that fit those goals. Below are some of the ones that I have seen done or practice myself. Also, be sure to check your HR policies since hopefully they already have a number of policies in place that try to achieve these goals (and with any luck the manager in question is violating them and at the very least can be reprimanded for breaking them).
Visibility into Managers Offices
With my current company our HR policy requires all managers offices to have a full sized unobstructed window into the office. The reason is so that when a manager needs to have a one-on-one with an employee and closes the door people in the work place can see what is going on in the manager's office. This helps protect both the manager and the employee from possible rumors and false accusations.
If a manager is trying to make inappropriate contact with a subordinate and a person comes walking by and sees it, then it is no longer a case of he said she said. Likewise if the the employee falsely claims that their manager was doing something inappropriate, the manager can point out that no one walking by saw anything.
Open Door Policy
As a manager and as a coworker make sure your subordinates and coworkers trust you enough to communicate problems and concerns with you. The sooner a problem is communicated the faster and easier it tends to be to fix. Also, having a strong professional relationship with your subordinates and coworkers makes it easier to communicate defensive strategies and encourage them to stand up for themselves.
Open Door Policies for Rooms
In this case, I am referring to any room that has no clear visibility into it when the door is closed. This can be anything from conference rooms to server rooms. If such a situation arises where exactly two employees are in said type of room the door must always be left open. The reasoning for this is the same as why manager's offices should have windows, but since said rooms do not have windows, leaving the door open accomplishes the same goal.
Inform Employees of HR Policies
This may seem like stating the obvious, but it is very important. How many of your employees know why offices have windows in them, or know of techniques or strategies to avoid being in a situation that could prove compromising? How many of them are actually following those strategies?
Once I had to explain to a young female coworker who was straight out of college why I propped the door to the noisy server room open when I entered and saw she was already in there. She was naive to the situation that by us being alone in that room it created that could prove compromising, and by doing a simple action of propping a door open that it could be mitigated. Since then I have spotted her propping the door open when she has been in there with other coworkers.
There is a phrase "knowledge is power." If your employees know these things and are properly on their guard it can help create an environment where sexual misconduct is hard to do without getting caught.
Even if you are working for a company where you are confident that there are no problems with sexual misconduct it is still a good idea to leverage these types of things to create an environment that makes it hard for sexual misconduct to become a problem. If your company's HR does not have these kinds of policies push hard for them. These things help protect employees, leadership, and the company from a variety of problems.
These types of things also apply to outside of the workplace too. There is a reason why Boy Scouts of America has a Youth Protection Policy and it contains a section about creating barriers to abuse with things like:
One-on-one contact between adults and youth members is prohibited. . In situations requiring a personal conference, such as a Scoutmaster conference, the meeting is to be conducted with the knowledge and in view of other adults and/or youth.