What is the ruling on not concealing a friend who was seen with a woman whose conduct is not good in his home without his wife being present?

It would have been better to conceal your brother's fault and advise him secretly regarding the impermissibility of being alone with a foreign woman, and that it is a step towards Satan leading to adultery, citing the verse: "And do not approach unlawful sexual intercourse. Indeed, it is ever an indecency and is evil as a way." As for informing your friend about what you saw, there is no harm in it if it is for the purpose of seeking help to change the evil, otherwise there is no need to inform him.