Is a young man who abandons his friends—who pray but persist in sins—sinful, out of fear of backsliding, and does this fall under the Hadith concerning a Muslim abandoning his brother Muslim?

You did well by staying away from bad companions, for a person's rectitude begins with that, and associating with them might infect you with their contagion. The Messenger of Allah (peace be upon him) said: "A person is on the religion of his intimate friend, so let one of you look at whom he befriends." The Muslim is required to stay away from sins and their causes, in fulfillment of the saying of Allah the Almighty: {O YOU WHO HAVE BELIEVED, DO NOT FOLLOW THE FOOTSTEPS OF SATAN}. This avoidance is not considered bad companionship or forbidden abandonment, because blameworthy abandonment is for personal motives. As for abandoning one whose state is blameworthy due to innovation or transgression, or if it benefits the religion of the one who abandons or the one abandoned, then it is not forbidden, as the Prophet (peace be upon him) did when he abandoned Ka'b ibn Malik and his two companions. Therefore, the hadith about a Muslim abandoning his brother does not apply to you, because your abandonment of them was to preserve your commitment and your religion.