I was 11 years old when my parents divorced and that was the best father's decision for me. I didn't understand thereat that my mother was addicted to antidepressants and needed professional help, but I remember the day when she was completely out of her mind and started yelling at me for the spilled juice on the table, and then she rushed at me with her fists, because it seemed to her that I wasn't sorry enough for this mistake. I hardly remember what happened next, but when I was packing my things to move with my dad to his parents' house, I heard her screaming at him, breaking the dishes and threatening that he would never see me again.
After 5 years and two rehabilitation of my mother, I was able to forgive her and now we are on good terms, partly thanks to my dad, who practically insisted that I should see her, that this could help bring back the mother I remember before tablets.
If he began to turn me against her, then I'm sure that this childish fear and resentment against my mother would ruin my life for a more long time.
Talk to your granddaughter's mother. If she doesn't want to steal her daughter's childhood and spoil her relationship with her father (any psychologist will prove to her that this is very important for girls), then it would be right to adhere to the general version of the reason for the divorce of the girl's parents - people develop in different ways, grow, change and one more they aren't good together. Therefore, people make a difficult and painful decision for both - to divorce. But this doesn't change the fact that she is their daughter and they love her.