Is a person's statement about his friend, "He practices masturbation," considered an accusation of unchastity against a chaste person?

Accusing someone of masturbating is not a legal accusation of qadhaf (slanderous accusation of fornication), because qadhaf is the accusation of zina (unlawful sexual intercourse). However, this statement falls under backbiting (gheebah), as it involves exposing a Muslim's private matters. One should repent to Allah and refrain from speaking ill of Muslims.