dnd 5e – Should Greater Restoration work against Prismatic Spray’s Indigo Ray?

My group is playing DnD 5e. We encountered an ancient blue dragon who cast Prismatic Spray at us. Our Ranger/Druid, who was in Air Elemental wildshape at the time, got struck by the Indigo Ray and failed his Dexterity Save:

6-Indigo: On a failed save, the target is Restrained. It must then make a Constitution saving throw at the end of each of its turns. If it successfully saves three times, the spell ends. If it fails its save three times, it permanently turns to stone and is subjected to the Petrified condition. The successes and failures don’t need to be consecutive, keep track of both until the target collects three of a kind.

Because he was an Air Elemental, he was immune to being restrained, but the DM had him continue to make making Con saves for the 2nd half of that Ray’s effects. Just before he (unknowingly) failed the third Con save, he transformed back into his usual Eladrin form, and became a Petrified stone statue as per the spell’s effects.

Upon defeating the dragon, the group attempted to restore him to his usual form using a Greater Restoration spell that he had stored in a Ring of Spell Storing. The spell successfully removed the Petrified condition, but because the spell says the creature “permanently turns to stone”, our DM ruled that he was now a living stone statue, unless/until he could find some other way to return him to flesh-and-blood status. This transformation came with a -2 to Dex (his main stat) but a +1 to Strength and Con, and some damage resistances. He’s not altogether miffed by these changes and likes the flavor of the ruling, but I’m wondering, should Greater Restoration have worked on its own to return him fully to flesh and blood, even if the spell says “permanently”?