How can victims of a Devourer / Spawn of Kyuss be restored?

True Resurrection, Wish, or Reincarnation

As pointed already a True Resurrection will restore a character who has become undead into their original form - so long as the creature has neither been dead for more than 200 years nor died of old age (PHB, p.284).

Interestingly, a Wish spell might or might not restore a creature who was turned into undead. It will be up to the DM to decide. The spell specifically says that it "is to duplicate any other spell of 8th level or lower." Note that True Resurrection is a 9th level spell (PHB, p.288).

Still, a Wish can achieve something beyond the scope of the examples provided in the spell, or duplicating an 8th-level spell or lower. It states: "State your wish to the GM as precisely as possible. The GM has great latitude in ruling what occurs in such an instance; the greater the wish, the greater the likelihood that something goes wrong." It either works, fails, or there is an unexpected result. This is up to the DM.

For example, Panny the fearless barbarian of the South has unfortunately been stolen away in the night, killed and has returned to the campsite in the dead of night, as a Ghoul! The party capture him and bind him, while deciding what to do. The wizard Parrhyus decides to use a Wish spell to bring Panny back and says: "I wish Panny is no longer undead and is brought back to life." >>poof<< Panny's soul agrees to return. However, the DM describes what happened. Suddenly, Panny's undead form began to shrivel until nothing is left but a pile of dust. From the dust, a plant begins to grow, and within minutes it becomes a magnificent 100-foot tall tree. Upon closer inspection, Parrhyus notes that there is a carving in the tree. He exclaims "Drat!" as he reads it: Panny.

Also, it's worth bearing in mind that using a Wish in this way will reduce the caster's strength for 2d4 days and, if they cast spells before having a long rest, they will also sustain necrotic damage.

As, as an additional option, you could try to use Reincarnate instead. However, the creature cannot have died more than 10 days prior. Unlike the 7th-level spell Resurrection there is no restriction mentioned about whether the creature is undead or not. So, as long as the character has not been undead for more than 10 days, this can work.

Alternatively, you could ask all your party members to provide you with piece of flesh of themselves, for you to keep in a safe place. Once a character dies, that bit of flesh becomes "a piece of a dead humanoid", which means that you can use it to cast Reincarnate and restore them to life - albeit in a different shape. Bear in mind you will need to have the material components with you, worth 1000gp (PHB, p. 271).

True resurrection.

A devourer creates either a zombie, ghoul, or wight, all undead; and a spawn of Kyuss creates another spawn of Kyuss, also undead.

You should kill or destroy the undead created by the devourer or spawn of Kyuss, and then cast true resurrection on the body or by speaking their name, whichever is appropriate.

True resurrection says:

You touch a creature that has been dead for no longer than 200 years and that died for any reason except old age. If the creature's soul is free and willing, the creature is restored to life with all its hit points.

This spell closes all wounds, neutralizes any poison, cures all diseases, and lifts any curses affecting the creature when it died. The spell replaces damaged or missing organs and limbs. If the creature was undead, it is restored to its non-undead form.

The spell can even provide a new body if the original no longer exists, in which case you must speak the creature's name. The creature then appears in an unoccupied space you choose within 10 feet of you.

True resurrection is quite clear, "If the creature was undead, it is restored to its non-undead form", since the devourer and the spawn of Kyuss both create undead from your fallen comrade, true resurrection will work to bring them back to their previous form.

Now, what if for some reason your 17th-level or higher party of adventurers cannot come up with 25,000 gp for the diamonds required for true resurrection? We are back to needing a wish spell - we still need not depend on the whims of our DM. Wish can create the diamond we need:

You create one object of up to 25,000 gp in value that isn't a magic item. The object can be no more than 300 feet in any dimension, and it appears in an unoccupied space you can see on the ground.

Then we can use that diamond to cast true resurrection. No need to worry about a scheming DM throwing some monkey's paw at our wish.