Asking for advice on talking to my dad about secrecy and loyalty

Assalamu alaikum everyone, I'm going through a really tough family issue and need some honest Islamic guidance. After being apart from my dad for about 11 years, we reunited here in the US. During that time, I found out that old accusations against him were actually true. He was involved somehow with a woman linked to our extended family. Two female relatives confronted him and tried to warn my mom, but he denied it all. My family ended up convincing my mom those relatives were lying. Later, after coming to the US, I came across enough info to believe they were telling the truth. I also learned my dad still keeps in touch with that woman. My mom's only request was that he completely cut contact with the relatives tied to this because it hurt her so deeply. But my dad still visits or helps them without telling my mom, knowing it would upset her. His help might genuinely come from a good place, but the secrecy and ongoing connection really pain her. Sometimes she finds out about his visits through other relatives. I haven't confronted my dad yet. I felt awkward, was busy with my studies, and scared of breaking up my family. Now he's getting ready to return to my mom, and I want him to treat her with total loyalty, honesty, and mercy for the rest of their lives. I don't want to spill everything to my mom because I think it would shatter her heart and maybe end their marriage. But I also feel guilty staying quiet while my dad keeps hiding things from her. Would it be Islamically okay for me to talk to my dad privately and respectfully before he leaves? I want to remind him about sincere tawbah, honesty, being loyal to his wife, and proper boundaries with non-mahram women. I don't plan to insult, threaten, or expose him-just encourage him to be a better husband and Muslim. Am I responsible for telling my mom what I know, or should I advise my dad privately to avoid exposing a past sin? Where's the line between covering someone's sin and becoming part of an ongoing deception? I'd really appreciate answers rooted in the Qur'an, Sunnah, or trusted scholarly sources. Jazakum Allahu khayran.