At the end of every 30 days, the creature can repeat its throw against this spell (phb240). Also, the part about Greater Restoration, Heal and wish ending the spell.Which part of the description?
It gives me the impression (as saves are an action of free will) that if the spell ends the NPC would be able to retry saves on any other spells cast on them. It is a DM ruling and I would have to think about it but as a DM it would be interesting story telling and fun to have the NPC fake it for a time.
