I see. Thank you for providing this additional information, Sheryl. Unfortunately, I hear about situations where the judge shows bias in favor of one of the parents frequently and that is difficult to fight.
There is no specific age wherein a child can testify in court or choose which parent to live with typically. A court can take into account a child's wishes if the court judges the child to be of sufficient maturity, but it is often best to have third party documentation or testimony which sets out the best interests of the child.
Here is a link which provides the factors courts can typically consider in determining the best interests of the child when deciding whether to award custody/visitation to a parent under MN law:
A third party professional can include a therapist, a school counselor, a guardian ad litem (GAL), or even official from child protective services (CPS).
The child could call CPS and make a complaint about the abuse involve while with the father. If CPS determines there are grounds to limit the father's visitation rights, they could aid the mother in gaining sole custody
and limited or no visitation by the father.
Another option is for the mother to request that a GAL be appointed to assess the best interests of the child. A GAL is usually a specialy trained attorney who will interview the child and other interested parties and make a recommendation to the court. If the GAL recommends that the father's rights be limited, that would typically aid in a modification of the father's visitation rights as well.
Given that there appears to be a biased judge involved, it would normally be best to have the strongest case possible by providing evidence from CPS, a GAL, and a therapist who has treated the child as well. If the court were to rule against the mother after being presented with such persuasive evidence, then there would be good grounds to appeal the decision if that becomes necessary.
I can't lie to you, it is an uphill battle when dealing with a biased court unfortunately, but the more evidence which can be presented to show that the father is abusive, the more likely the court will be to rule in the mother's favor despite their possible bias.
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