How can guidance and interpretation be offered to someone who was a Muslim, well-versed in the Qur'an and Sunnah, but apostatized a few years ago?

One should repent and return to the religion, for that benefits one in this world and the Hereafter. As for disbelief and atheism, they destroy their practitioner and corrupt their livelihood and their ultimate destination. One must seek refuge from Satan, who beautifies disbelief. Allah Almighty says: "Indeed, Satan is an enemy to you; so take him as an enemy. He only invites his party to be among the companions of the Blaze." Striving against Satan includes repelling doubts and suspicions that undermine faith, and repelling corrupt desires and lusts. The first leads to certainty, and the second to patience. One must also identify the cause of falling into atheism and address it, while being patient, acquiring beneficial knowledge that repels doubts, performing righteous deeds that protect against the evil of lusts, and keeping good company that aids in goodness.