Is it obligatory to honor a father who has sexually abused or been violent towards his children, or how should his children deal with him?

You should highlight for this woman the virtues of the Islamic religion, among them Islam's command to honor one's parents and its prohibition of indecency, especially with mahrams (unmarriageable kin). Despite the father's misconduct, his wrongdoing does not negate his right to filial piety and good treatment from her. Honoring him remains obligatory, even if he were a polytheist. Among the greatest acts of filial piety towards him is to supplicate for his repentance and guidance, and to advise him with wisdom and good admonition.