Sins are of two types:
1 – Those which constitute kufr and put a person beyond the pale of Islam – we seek refuge with Allaah. The one who commits such sins is a kaafir who is beyond the pale of Islam if the conditions for describing him as a kaafir are met and there are no impediments to doing so – such as one who associates others in worship with Allaah, or does not pray at all, and so on.
2 – Sins which do not constitute kufr and do not put a person beyond the pale of Islam, but the one who does them is described as faasiq (rebellious evildoer) and a believer who is lacking in faith, such as zina (illicit sexual relationships), drinking alcohol and so on, if he does not regard these actions as permissible. If he does regard them as permissible, then this puts him beyond the pale of Islam, if the conditions for describing him as a kaafir are met and there are no impediments to doing so. The belief of Ahl al-Sunnah wa’l-Jamaa’ah and the consensus of the salaf is that a person who commits a major sin but does not regard it as permissible is not to be regarded as a kaafir. For more information see the answer to question no. 9924.
Once this is understood, then the ruling on making friends with people is based on the above.
It is not permissible to take kaafirs as close friends, or to mix with them and feel friendly towards them, or to live amongst them and take them as friends, or to love them, or to prefer them over the believers and so on. Allaah says (interpretation of the meaning):
“You (O Muhammad) will not find any people who believe in Allaah and the Last Day, making friendship with those who oppose Allaah and His Messenger (Muhammad), even though they were their fathers or their sons or their brothers or their kindred (people). For such He has written Faith in their hearts, and strengthened them with Rooh (proofs, light and true guidance) from Himself. And He will admit them to Gardens (Paradise) under which rivers flow, to dwell therein (forever). Allaah is pleased with them, and they with Him. They are the party of Allaah. Verily, it is the party of Allaah that will be the successful”
But we are obliged to treat them fairly and justly, and not wrong them or commit acts of aggression against them with no shar’i reason. It is permissible to interact with them by buying, selling, lending, etc. It is narrated in saheeh reports that the Prophet (peace and blessings of Allaah be upon him) borrowed a weapon from Safwaan ibn Umayyah, and that he bought food from the Jews.
With regard to sinning Muslims, we should love them according to the faith that they have, and hate them according to the sins and immoral actions they commit, but being friends with them goes against the words of the Prophet (peace and blessings of Allaah be upon him): “The likeness of a good companion and a bad companion is that of a perfume seller and one who works the bellows. With the perfume seller, either he will give you something or you will buy something from him, or you will notice a good smell from him, but with the one who works the bellows, either he will burn your clothes or you notice a bad smell from him.” Narrated by Muslim, no. 2628.
But if the intention behind sitting with this person is to soften his heart and call him to Allaah, and guide him to the right path, then this is a virtuous action, as Allaah says (interpretation of the meaning):
“And who is better in speech than he who [says: “My Lord is Allaah (believes in His Oneness),” and then stands firm (acts upon His Order), and] invites (men) to Allaah’s (Islamic Monotheism), and does righteous deeds”
But this is subject to the condition that he should not have any effect on you or cause you any harm when you are seeking good.
Based on the above, if the person you are asking about does not pray at all, then he is a kaafir, because the most correct scholarly view, which is the view of the salaf, if that the one who does not pray is a kaafir in the sense of major kufr, and is beyond the pale of Islam. For more information on the evidence on that, please see questions no. 2182, 5208, 6035, 33007, 10094. Based on this, it is not permissible to befriend him or be close to him, rather he should be called to repent to Allaah and pray regularly. Allaah says (interpretation of the meaning):
“But if they repent [by rejecting Shirk (polytheism) and accept Islamic Monotheism], perform As-Salaah (Iqaamat-as-Salaah) and give Zakaah, then they are your brethren in religion”
It should be noted that keeping company with good people is something enjoined by our Lord, may He be exalted, and our Prophet (peace and blessings of Allaah be upon him), as mentioned in the hadeeth quoted above. And Allaah says (interpretation of the meaning):
“O you who believe! Fear Allaah, and be with those who are true (in words and deeds)”
“And keep yourself (O Muhammad) patiently with those who call on their Lord (i.e. your companions who remember their Lord with glorification, praising in prayers, and other righteous deeds) morning and afternoon, seeking His Face”
And Allaah knows best.For more information see Fataawa al-Shaykh Ibn ‘Uthaymeen, 3/31.