Case:

Ruling : The mishna (Sanhedrin 27b) cites a machloket between R. Yehuda and the Chachamim if a witness can testify when a litigant is a friend or an enemy of his. The gemara (ibid. 29a) says that even according to Chachamim who permit it, such a person cannot serve as a dayan. Another gemara (Ketubot 105b) says: "One should not judge one whom he loves or one whom he hates: the one he loves, because he will not see his fallacy, and the one he hates because he will not see his virtue."
Tosafot (ad loc.) is bothered by the fact that the former gemara is based on a pasuk, whereas the latter one does not seem to be. They posit that there are actually two levels of friends and enemies. A close friend or serious enemy is so unfit that if he did take part in the case, his ruling would be invalid even after the fact. Mild friends or enemies should not hear the case, but if they ruled, the ruling is valid after the fact. According to the Beit Yosef’s (Choshen Mishpat 7) understanding, the Rambam (Sanhedrin 23:6) does not distinguish between different levels of friends and enemies; rather, the ruling of anyone who should not serve for this reason is invalid even after the fact. This is also the view of the Ohr Zarua (see Haghot Ashri, Sanhedrin 1:6). The Shulchan Aruch (CM 7:7) rules like the Rambam, whereas the Rama (ad loc.) rules like Tosafot. The Maharik has a third opinion. The relatively weak level of relationships that are described in Ketubot do not strictly invalidate a dayan but just create a midat chasidut (preferable course of action) to avoid. In a case where by following the higher level of carefulness one of the litigants will lose out, he says that one should not disqualify himself.
It is impossible to say that, in our case, there is a high level of friendship. Beit din did not even feel that there was even a level of friendship that could disqualify the av beit din at all (according to Tosafot). It is possible that it would have been better for the dayan not to have taken the case as a midat chasidut. However, considering that it is a case that is toward the end of its deliberations and a change will cause a damaging delay, the dayan has no right to remove himself from the case, along the lines that the Maharik stated.