What is the ruling on befriending irreligious people, and is it forbidden to keep in touch with them to check on them? What should one do if abandoning them might increase their deviation, and they are unresponsive to advice?

It is not permissible to associate with transgressors and take them as friends, for companionship is only permissible with those who obey Allah. The jurists have agreed that it is not permissible to befriend a transgressor because of his transgression, nor to sit with him while he commits sins in a spirit of camaraderie or compliance, due to the Almighty's saying: {And do not incline toward those who do wrong, lest the Fire touch you}, and the Prophet's (peace be upon him) saying: "Do not accompany anyone but a believer, and let no one eat your food except a pious person," and "A man is upon the religion of his intimate friend, so let one of you look at whom he befriends."

Sitting with transgressors while they are committing sins is not permissible. However, sitting with them or contacting them in circumstances other than that is not forbidden, especially if the intention is to reform them, enjoin good upon them, and forbid evil from them. But if they do not respond, then they should be shunned.