What is the religious ruling on dealing with a father who talks to another woman on the phone: should the father be confronted about it, or should the mother be informed, knowing that he has a beard, has performed Hajj, and is committed to providing for the family's needs?

It is not permissible to disclose what the daughter witnessed from her father, based on the Prophet's (peace be upon him) saying: "Whoever shields a Muslim, Allah will shield him on the Day of Resurrection." However, he must be advised and warned of Allah's punishment, based on the Prophet's (peace be upon him) saying: "Religion is advice," and "Help your brother, whether he is an oppressor or oppressed." The advice should be given gently and kindly. If that does not work, there is no harm in threatening to inform the mother.