Forgive & Forget???

Is it necessary to disown your mother if he cheated on your father? My grandmother is few years younger than my grand father, beautiful, socialite, bubbly & etc. She's stays with her children & cheating with his husband only my father knew about that affair & keeping it from my grandfather in order not to cause a broken family. But once my grandfather knew about it my Father gets all the blame & in up disowning his mother. His life became miserable after it happened. If this happens to your family what will you do? Will you still forgive your mother & forget?