All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
It is not permissible for a woman to work in a place that exposes her to be in suspicious seclusion and mixing with men as clarified in Fatwa 83607. It is incumbent upon that woman to repent to Allaah The Almighty from her negligence in this regard. Repentance is fulfilled by giving up the sin, regretting doing it, and resolving not to go back to it. Accordingly, she should leave that job and if she really needs to work, she should search for a lawful work that does not expose her to be in seclusion with men. For more benefit, please refer to Fatwa 86527.
As for the acceptable means of judicial testimonial evidence, you can refer in that regard to the court.
Also, that sister should not reveal what happened to her to anybody, and there is nothing wrong with consummating her marriage after making sure that she is not pregnant (by observing one menstruation). She is not obligated to tell her husband about the reason for losing her virginity. For more benefit, please refer to Fataawa 92632 and 180055.
Allaah Knows best.