How does one deal with a person who shows hypocrisy and reneges on their commitment to advice after having accepted and acted upon it?

You must advise your friend again and not despair, for hearts are between two fingers of the Most Merciful's fingers, and you will be rewarded for your advice. Al-Manawi said: "And the apparent meaning of the hadith is the obligation of giving advice even if it is known that it will not benefit the advised person." Be patient in your advice, as Allah Almighty said through Luqman: {O my son, establish prayer, enjoin what is right, forbid what is wrong, and be patient over what befalls you. Indeed, that is [part of] the affairs [requiring] determination.} [Luqman: 17]. Make your advice with good admonition and kind words, avoid advising him in front of others, and choose appropriate times and means. Do not rush to judge people as hypocrites, for they may have excuses.