I am going to stick my neck out and put another angle, from the perspective of a family I am close to. The story starts in a similar way.
The Mum had long term health problems, including mental illness. Marriage broke up, Mum left, moved away. Girl stayed with Dad. At last, some stability for the child, and good support from her Grandparents, her Dad’s parents.
Mum failed to keep contact, Unstable personality, poor health. Did not turn up to meetings arranged, and as a result, her parents lost contact with the child too. The distance was too great for them to travel to see the child, and Dad was not keen on taking child to stay with them as it would reawaken the difficult relationship with her mum.
Although child had a happy and stable childhood with her Dad after the breakup, and a good education, twenty years on she too is unable to lead a full adult life, with severe mental health problems. Her Dad is the only person she trusts.
There are no easy answers.