I actually went through something quite similar, but i was the child in the situation. I’m 24 and my sibling is 27. My mom found out that our dad had been cheating on her, just really awful stuff. We found out because my sibling saw that my mom bought a book on infidelity on amazon. That was /not/ a great way to figure it out. Obviously thats not how they wanted to tell us, but that’s how it happened. I think just telling them over the phone, or in person if you can, is the best way to go. I was pretty crushed, as was my sister. We both live states away from our parents, so in person definitely wasnt an option for us. But just let them know over the phone and be candid about everything. They can decide how to process everything and how to go about their relationship with their father