The devil is a fallen angel. Because they are pure spirit and because of their superior intelligence, the angels made an irrevocable decision for or against God. Those who chose against God have by that choice permanently and eternally separated themselves from God. Because the choice of the angels is irrevocable, it cannot be changed. Thus, there is no point in praying for the devil: He is incapable of "changing his ways."
Humans make their final irrevocable decision at death, so that is why we can pray for them in this life. Because their final choice for or against God is unknown, we can pray for them after death. So although we need not pray for the repose of the souls of angels, we can and should pray for the repose of the souls of human beings.
Should we love the devil?