Answer #1 (Rav Sheshes): She married one of the witnesses
that say that he died (since he is sure she is permitted,
he may remain with her).(88b)
HOW IT COMES? IN THE SECOND PEREK in yevamot (in the mishna) we learn already that the woman is prohibited to marry a witness who says that her husband died (even if he addmitted that he killed him)?
In the Mishnah earlier (25a), the Mishnah is referring to a case where only one witness testifies. Here, though, the Gemara is referring to a case where two witnesses testified that her husband died. In such a case, she is permitted to marry one of the witnesses, because we do not suspect that two witnesses will collaborate and lie in order to enable one of them to marry the woman.
Yisrael Shaw