Is what the questioner said to his friend—that the wife of a married man is speaking to a young man on the phone—considered slandering chaste women, and how does one repent from that if it is slander?

A woman about whom offensive things are said must be admonished, and the seriousness of her actions must be explained to her. If she does not stop, she can be threatened with informing her husband. What you mentioned to your friend about the woman is not slandering chaste women, but rather backbiting, which is forbidden and permissible only in specific cases and for a legitimate purpose.