Father Seraphim of Platina, may his memory be eternal, has taught that aliens are diabolic.
I don't think that's precisely accurate. As I understand it, he taught that unexplainable UFOs are illusions or phenomena created by demons for the reason of distracting people and essentially directing their spiritual energies into ufology and alien paraphernalia, rather than searching for the truth and finding Christ. Maybe it's six of one and half-dozen of the other, but there is a distinction IMO.
To the OP, I personally don't believe there are other sentient beings out there. Rare Earth Hypothesis and all that. Certainly there are none skimming the skies of our humble planet.
But for argument's sake, I would say their salvation would be wrought the same way as the rest of Creation—by Christ's life, death, and resurrection.
Human disobedience corrupted the Cosmos. For this reason, God does not need a different plan of salvation for every other sort of creature. Christ did not come as a dog to save dogs, or as a juniper bush to save juniper bushes. He came as a Man, and by God becoming Matter, all Matter was transfigured.
All of the Cosmos will be finally and ultimately reconciled to God when the New Heaven and the New Earth are once again joined at the end of time. This includes the dogs and the juniper bushes, as well as the astrobiology.
Perhaps the Church could invent some kind of rite for sentient non-humans to express their need to feel a part of the salvation of the universe, as a pastoral measure. But I think telling them they are guiltless in the matter would be pretty nice to hear. To baptize an alien into the Church would be like muzzling your cat because your dog is biting people. The aliens haven't caused the universe to decay, so they have no reason to be part of the Church.