I have found male staff in the local unit where I have been many times, very good. If they are going to talk with somebody esp females on their own, they take the person into one of the sitting rooms, the courtyard, the OT room, the foyer with armchairs quietly this means they are not entirely alone with someone but they have the privacy to speak with someone but are within view of other staff and cctv.
They dont enter the female bedrooms to speak to patients alone. However the bedroom door has to be wedged open with a shoe at night so they can do their obs, or when on specials 1-2-1 they have switched over with female staff when it came to personal stuff like having a shower (usually after an argument about having it).
Otherwise they will sit at the doorway of the room and watch the patient, only entering if absolutely needed if the patient is about to harm themselves and by that point will hit the buzzer 1st so there is staff hot on their heels to help diffuse the situation.
The obs window in the door has to be open during the day except when changing clothes or getting dressed and patients are encouraged to do that in the en-suites so that there is no reason to touch the obs windows.
However, there is still the potential that your friend may be abused. I would as pp advised, for now, take as many notes of what your friend says as you can.
Wait until she is well and let her tell you how things are or if she wants to touch back on that subject. Support her then to access other help and report as needed.
It also means she is out of the situation and while her potential abuser might be suspended the fall out may impact her while in the unit from the other staff.
If she is also paranoid and delusional right now, then she may not be listened to properly while she is ill as it may be put down to aspects of her illness and when she is well, she can take the actions as needed and gain support for this without impacts from other staff affecting her.
Hugs to you as it is hard to watch your friend be so ill and trying to support her as best you can, be sure to take best self-care of you too.