Case:


Ruling: The Shulchan Aruch (Even Haezer 42:4) says that a marriage is valid without the principals specifying witnesses, and the Rama adds, in the name of the Rivash, that even if one specified witnesses, others who saw the act of marriage can testify. There are two ways to understand the Rivash. According to one, even if the chosen witnesses were invalid, others who were present can serve as witnesses to validate the marriage. If this is so, Dina’s marriage was valid. However, the Rivash might mean that although only those who were chosen to testify can make the marriage valid, others can testify in court that they saw a valid marriage accomplished with the help of the appointed witnesses. According to the second approach, Dina’s marriage depended only on the two witnesses, and turns out to be invalid when one proved to be unfit.
The Mahari Weil (7) follows the second approach. He says that once witnesses are singled out, only they can serve as witnesses, and any valid witnesses who happen to see the marriage cannot be used to enable it to take effect. The Rosh (Teshuva 60:3) says that when one of the specified witnesses for a will was unfit, other kosher witnesses present can salvage it. However, it is unclear why assigning kosher witnesses excludes invalid witnesses (for, if included, they would disqualify the set of witnesses they are a part of - see Makkot 6a), while assigning non-kosher ones does not eliminate kosher ones.
It appears that when one specifies fit witnesses, we understand that he intends to exclude non-kosher witnesses, who, if included, would destroy the testimony. In contrast, when one specifies witnesses and it turns out that one of them is invalid, there is no reason to think he wants to invalidate kosher witnesses who can salvage the testimony (see Chavot Yair 19).
The Kerem Shlomo (21), in discussing the Rivash, does assume that if the ba’al davar (the person who is doing the process) specifies witnesses that he thought were kosher, others are excluded even if one of the assigned witnesses was invalid. One can explain this approach by saying that even if he has no reason to exclude kosher witnesses, it his words, and not his thought process, that determines matters. However, if it is the officiating rabbi who chose the witnesses, then in a case where his choice turned out to be a bad one, we would discount his ill-advised act of agency and allow kosher witnesses to be included (see Shut Chatam Sofer, EH 100).
In the final analysis we do not have sufficient justification to undo Dina’s marriage to Reuven.