What is the way to help my sister who is suffering in her marriage to a spendthrift, philandering, gambling-addicted, deceitful man who abandons prayer, knowing that she is committed [to her religion] and has a child with him, and she is now performing Istikhara prayer and is torn between staying or divorcing for fear that her husband might take her son from her?

If the husband neglects his duties and mistreats his wife, she has the right to request a divorce. If he refuses, she can take the matter to a judge to compel him, as a wife has the right to divorce due to clear harm. It is also advised that the wife seek separation from him through divorce or khul' (divorce at the wife's instance, usually by returning the mahr), because her continued presence with him will only worsen her situation. As for her fear of him taking the child, the mother has a greater right to the custody of her child unless she has a disqualifying factor, or she marries another man, until the child reaches seven years of age, at which point he is given the choice between his parents.