Abuse - Testimony only to prevent future attacks

Shulchan Aruch(C.M. 28:1): Whoever knows testimony regarding another person and he is fit to testify and the other person would benefit the other person – he is obligated to testify if he is requested to testify by the court. It doesn’t matter whether there is another witness that will testify with him or not. However if he refuses to testify he is exempt from human punishment but he is liable to heavenly punishment. Rema: A single witness should not testify except regarding monetary issues which will bring about taking an oath or concerning something which is prohibited and it will cause the person to stop sinning. But if the person has already done the sin he should not testify because then it is simply motzi shem rah (slander). It is prohibited for a person to testify about something he doesn’t know directly even though the information was told to him by someone he knows does not lie. Even if someone asks him to stand next to a single witness and not to say anything in order that the one who owes him money will think that he has two witnesses and therefore confess – he should not listen to such a request.