Does informing one of my friends that I repeatedly saw a girl talking to a young man without a legitimate relationship between them count as slandering chaste women?

The basic principle is that a Muslim should advise his Muslim brother who is discreet [about his sins] if he commits something forbidden. However, if he repeatedly commits the forbidden act and does not desist, and the reprehensible act is serious, such as fornication (zina), then the matter should be referred to the ruler.

It would have been preferable to advise the young man and woman directly. If they do not stop, then the girl's father should be informed of his daughter's behavior, if he is receptive to such information.

Merely informing others about this sinful relationship without explicitly accusing them of fornication (zina) is not considered slander (qadhf). Slander (qadhf) is the denial of someone's lineage or accusing them of fornication (zina).