What is the ruling on the actions of my friend who speaks on the phone with affectionate and flirtatious talk and sexual conversations with a married woman, and meets her to embrace and kiss her, while her husband is traveling? And what should I do so that he avoids falling into full adultery?

Your friend's relationship with that woman is forbidden due to his transgression of Allah's limits and his following the footsteps of Satan. Continue to forbid him from this evil and remind him of Allah and His punishment. If he does not repent, threaten to inform someone who can deter him. If that does not work, abandon him and cut off your relationship with him, explaining that your abandonment of him is because of his disobedience, as reported from Hanbal: "If it is known that a man persists in disobedience, one is not sinful if he shuns him until he repents." Be keen to associate with the righteous, for the Prophet, peace and blessings be upon him, said: "A man is upon the religion of his intimate friend, so let each of you look at whom he befriends."