Is it permissible to follow one's father to ascertain whether he is speaking with a foreign woman, knowing that it is permissible for him to marry a second wife, or is it better to conceal his matter?

A Muslim must avoid ill-suspicion, spying, and seeking out the faults of others, especially parents. Concealing the faults of a Muslim is obligatory, and it has been narrated in the Hadith: "Whoever conceals [the fault of] a Muslim, Allah will conceal him in this world and the Hereafter." It is not permissible to investigate the father to confirm suspicions, for it has been said: "And if you suspect, do not investigate." If it becomes clear that the father's action violates the Sharia, then he must be advised gently, courteously, and respectfully, in accordance with Allah's saying: "So do not say to them [anything that indicates] disapproval and do not reprimand them but speak to them a noble word. And lower to them the wing of humility out of mercy." Ahmad ibn Hanbal said: "If he sees his father doing something he dislikes, he should inform him without violence or abuse, and not be harsh in speech towards him, otherwise, he should leave it. And a father is not like a stranger."