I have repeatedly seen a girl speaking with a young man who has no legal relationship with her, and I told one of my friends about that. Is this considered slander against a chaste women?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

As a basic ruling, whenever a Muslim individual sees his fellow Muslim do something prohibited, he should advise, instruct, and conceal him, for the Prophet, sallallaahu ‘alayhi wa sallam, said:

"The Muslim is the brother of his fellow Muslim; he does not wrong him or let him down. Whoever fulfills his brother’s needs, Allaah will fulfill his needs, and whoever relieves a Muslim of some worldly distress, Allaah will relieve him of some of the distress of the Day of Judgment. Whoever covers a Muslim (conceals his sin), Allaah will cover him (conceal his sin) on the Day of Judgment."

If the person went on with his sins, then the one who knows about his attitude should advise and admonish him. If he does not give up and the sin is a grave one, in such a way that it can cause evil consequences, such as Zina (adultery/fornication) or the like, then the case should be referred to the ruler to deter him.

Accordingly, it was more appropriate to advise that young man and that young girl directly and to explain the prohibition of what they were doing to them. If they did not give up, then you had to inform the father of that girl about his daughter's conduct if you know that he may accept that from you.

Anyway, just telling some other person of that sinful relationship without explicitly stating that they practice Zina is not slander. That is because scholars defined slander as blemishing the integrity of lineage of the slandered or accusing him of Zina. Khaleel ibn Is-haq,  the Maaliki scholar said: “Slandering occurs when someone who is competent for religious assignment denies the ascription of a free Muslim individual to his father or grandfather's lineage.” Al-Hattab, in explanation of Khaleel's statement, said, quoting Ibn ‘Arafah: “The most general meaning of slander occurs when someone accuses another with Zina or denies the integrity of his lineage.”

Allaah Knows best.