Does the Almighty's saying, "Whom Allah sends astray—there is no guide for him," mean that Allah will not guide whoever has distanced himself from Him, or are these satanic whispers to prevent him from returning?

The mentioned verse indicates that for whomever Allah decrees misguidance, there is no guide for them other than Him. It is not necessary that someone who went astray during their life will have that as their end, for Allah's decree is a hidden secret. You must seek Allah's help and promptly embark on the path of those who repent, for everyone is facilitated for what they were created. Turn away from the whispers of Satan, and do not despair of Allah's mercy. Rather, hasten to repent, for Allah has commanded us to do so.