Boycotting someone who is not praying
Question

                Respected Sheikh,my question is, a person's relative is not performing his/her prayers(Salah),therfore he/she has left the fold of Islaam.Is it permissable to greet them? Or should they be completely ignored
Praise be to Allah.Not performing one’s prayers 
          is a dereliction of the most important duty among the pillars of Islam. 
          Prayer is the pillar and cornerstone of the faith, and it is not permissible 
          under any circumstances to be lenient towards the one who is not praying. 
          First of all his family should advise and enjoin him to pray, then they 
          should boycott him and turn away from him, and not greet him with salaam, 
          or eat with him or sit with him, in order to point out to him the seriousness 
          of sin, so that he will come back to Allaah and repent. The opinions 
          of the scholars with regard to the one who does not pray may be summed 
          up under the following two headings:  
         1.The opinion that the one who does not pray is a kaafir, which 
          is the opinion narrated from ‘Umar, ‘Ali and Ibn Mas’ood (may Allaah 
          be pleased with them). It was also the opinion of al-Hasan, al-Sha’bi, 
          al-Oozaa’i, Ibn al-Mubaarak and Muhammad ibn al-Hasan; it was also reported 
          from Ahmad, and is indicated by the hadeeth of the Prophet  
          (peace and blessings of Allaah be upon him), “All that stands between 
          a person and kufr is his not praying.” (Reported by Muslim, Ahmad, Abu 
          Dawood and Ibn Maajah). 
2.The opinion that the one who does not pray is not a kaafir. This 
          is the opinion of Abu Haneefah, Maalik, and al-Shaafa'i, and is indicated 
          by the hadeeth of ‘Ubaadah ibn al-Saamit, who reported that the Prophet 
           (peace and blessings of Allaah 
          be upon him) said: “There are five prayers which Allaah has prescribed 
          for His slave during the day and night, whoever does them and does not 
          wilfully neglect them, Allaah will be bound to grant him admittance 
          to Paradise. Whoever does not do them will have no such guarantee; if 
          He wills, He will punish him and if He wills, He will grant him admittance 
          to Paradise.” (Reported by al-Daarimi, 1531; Maalik, al-Muwatta’, 
          248; Ahmad, 21690). 
 However, according to both opinions, 
          if a person insists on not praying, he should be asked three times to 
          repent, and if he refuses, then his fate will be execution.