Thank you. Let me first assure you that the father has an uphill battle ahead if he wants to gain sole custody.
First, because there is already a custody order in place, the father would have to demonstrate that there has been some substantial change in circumstances since the last custody order. (If it has been less than 2 years since the last custody order, the father would have to file an affidavit stating that the child risks serious endangerment if left in the current custody situation). That in itself is a heavy burden, and you have not described anything which would lead me to believe that the circumstances have changed enough so that the court could review the current custody order.
Secondly, the father will have to demonstrate that the proposed change is in the best interests of the child. Rarely is it in the best interests of the child to change the custody arrangement when there has been no incident or change that would warrant that. Stability is a major consideration when reviewing custody, and the courts favor stability in terms of the child's home life, education, relationships, etc.
That said, contested custody motions should always be taken seriously, and your daughter should strongly consider hiring an attorney to defend against the request in order to ensure the most favorable outcome.
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