The Gemara discusses the case fro the Mishna on Daf 118a where a lady says testimony that her husband and then her father-in-law died. We don't believe her because she's trying to ruin her mother-in-law's life. Would this halacha apply if she gets married and only after that she says the testimony? Do we say once a mother-in-law she has that hatred and would always try to ruin her then ex mother-in-laws life?
The Din only applies when she is presently her daughter-in-law. We are afraid that even what she said about her own husband was a ruse. (Tosfos writes that even if the daughter-in-law gets married first we are afraid that sometimes the opposite may happen.)
However when she is already an "ex" at the time of the testimony there is no problem.
Dov Zupnik