I think we have to keep in mind that the girls may have begun to treat Ioan this way anyway, despite the divorce. He said himself that Alice belittled him in front of them and treated him with utter disrespect and encouraged the girls to do the same, from BIRTH.
All their lives they will have seen their dad undermined and treated as though he was stupid and a nuisance. They witnessed terrible arguments where their mother berated him, swore at him and them, and physically assaulted him. Before Ioan had even mentioned leaving, Ella was absolutely brainwashed to see her dad as the problem and her mum as the poor victim; we saw that in those heartbreaking transcripts of the abuse, with Ella desperately offering to be a therapist and fix her dad so her mum would be happy again. And what we know is the tip of the iceberg I'm sure.
We saw how she mocked him in private messages, and more publicly on Instagram. She ignored every boundary he had. We know the names she called him and the physical and financial abuse. We know that she thought so little of him that she suggested he live in the garage. And this is only the behaviour Ioan reported. We know that she will have been mocking and disparaging him to the girls the entire time he was working away. Even if Ioan had stayed forever, the girls would possibly have despised him as weak and all the things their mother told them he was just like they do now. All the problems of the household would have been blamed on him anyway. They would never have had financial security, as Alice spends everything that comes in the door and refuses to earn anything herself, and IG's mental health would have spiralled further, making work for him very hard too. With Alice as their main caregiver, I can't see an outcome that is any different, whether IG had stayed married to her or not.