Is the fasting of one who views pornographic images while fasting accepted? And is this considered a major sin? And is it permissible to distance oneself from him due to his unresponsiveness to advice?

Viewing pornography with lust is a major sin. The acts of worship of a person who commits sins are accepted imperfectly if he is a believer, for Allah accepts from the righteous, and Shirk (polytheism) is excluded by the consensus of the Ahl al-Sunnah. One must avoid the company of those who do not respond to advice, for the Prophet, peace and blessings be upon him, likened a righteous companion to one who carries musk, and a bad companion to one who blows the bellows.