Is it permissible to greet or ignore a relative who does not pray, and is such a person considered to have left Islam thereby?

Abandoning prayer demolishes one of the most important pillars of Islam. The one who abandons it should be advised and shunned. Scholars have differed concerning the ruling on one who abandons prayer: some deem it an act of disbelief, based on the Prophet’s (peace and blessings be upon him) saying, "Between a servant and disbelief is the abandonment of prayer." Others do not consider it disbelief, citing the hadith of 'Ubadah ibn al-Samit. Both groups agree that the one who abandons prayer should be given three chances to repent; if he does not repent, he is to be executed.