Should a friend be informed about being lied to in the past to clear the names of innocent women, with the potential consequence of losing the friendship, or should one remain silent while fearing God's punishment?

If the woman who was slandered is specified, then this is a grave slander and necessitates the hadd (prescribed punishment) for zina (unlawful sexual intercourse) upon you, even if you were lying. However, if she is not specified, then it is merely an admission of zina. The repentance of the slanderer is to recant his statement and inform his cousin that he was not truthful in his claim, in order to exonerate the woman. You should prioritize Allah's pleasure over your cousin's friendship, for whoever seeks Allah's pleasure, Allah will suffice him against the burden of people.