Is it considered backbiting and slander to reveal a secret about a friend who was in an illicit relationship—whose nude pictures I saw on his phone and from whom I learned of her suspicion of pregnancy—even though I did so out of sincere advice and concern for my female relative whom he proposed to?

If the suitor is still a transgressor (fasiq) and has not repented, then informing those who are considering him for marriage about his condition is a mandatory piece of advice, because "the religion is advice." It is permissible to warn against evil in a way that achieves the purpose without overstepping the bounds, such as saying: "He is not suitable for you," or "He had a previous relationship with a girl." As for slander (qadhf), it is a grave sin that necessitates repentance.