Must one divorce his wife if, after marriage, it is discovered that she had been speaking with a young man in explicit sexual phone calls and engaging in masturbation with him over the phone, and sending him pictures of her chest during the engagement period, in addition to her meeting another young man and hugging and kissing him before the engagement?

If it is confirmed that the girl committed reprehensible acts before marriage, then what matters is her current state. If she is now upright and of good character, then keep her and continue with her, treat her well, do not hold her accountable for what is past, cover her faults, and help her in matters of her religion by teaching her and guiding her well. It is your duty to protect her from causes of temptation by obligating her to cover herself and wear the hijab, and to avoid unlawful mixing, in accordance with the Almighty's saying: "O you who have believed, protect yourselves and your families from a Fire" (At-Tahrim: 6), and His saying: "Men are in charge of women" (An-Nisa: 34), and the Prophet's (peace be upon him) saying: "Each of you is a guardian and is responsible for his subjects... And a man is a guardian over his family and is responsible for his subjects."