Case:


Ruling : The Shulchan Aruch (Choshen Mishpat 28:1) rules: "Whoever knows testimony about his friend, is fit to testify, and it is a situation where his friend could benefit from the testimony is required to testify when he is demanded to do so, whether he has another witness with him or whether he is on his own."
There are three sources that justify this halacha: 1) There is a direct obligation to testify (see Vayikra 5:1); 2) There is a prohibition to stand idly by as one is damaged; 3) There is a mitzva to return a lost item. The latter two sources apply even to testimony in secular court, as a manifestation of the obligation to assist a counterpart in trouble. Regarding the formal obligation to testify, the Divrei Yatziv (CM 79) cites the Chik’kei Lev (CM 26) who leaves it as an open question if the mitzva to testify applies in secular court. Regarding a felony case, it is even possible that the governmental secular courts are the proper venue, in which case the Chik’kei Lev might agree that the mitzva to testify applies.
It is a question whether the obligation to testify applies to a single witness. The Shulchan Aruch’s language implies that it does. The K’tzot Hachoshen (28:3) says that it applies in regard to the mitzva to return a lost item. The Netivot Hamishpat (28:1) says that for one witness, there is only a rabbinic obligation.
The Shulchan Aruch (ibid.) says that even if one did not fulfill his obligation to testify, he is not obligated to pay for losses that resulted from this refusal. However, he has a chiyuv latzeit y’dei shamayim (a moral obligation) to compensate. In this case, there are specific reasons to exempt def from paying. Def told police who investigated the matter that both parties were involved in the physical contact. Thus, he already let law enforcement know that the event was not a one-sided attack, and the courts were already made aware of this. Furthermore, in his ruling, the judge said that he would have held pl liable for these payments even if he would have accepted fully pl’s version of the story. Finally, even if it were possible that the information that def withheld would have enabled pl to have won the case, he did not prove that claim to beit din.