Is the inquirer considered to have committed a sin if he does not advise his friend who does not pray and is negligent in prayer?

Abandoning prayer is one of the gravest evils. Denouncing evil is a collective obligation (fard kifaya) upon whoever knows about it and is capable of doing so. The sin is lifted from an individual if someone else performs it, but everyone incurs sin if no one denounces it. The questioner should promptly denounce this evil to the one who abandons prayer with wisdom and gentleness. If he persists, then know that there is no good in the company of one who abandons prayer, for the Prophet, peace and blessings be upon him, said: "Do not accompany anyone but a believer, and let no one eat your food but one who is pious."