Nat 20 doesn't just let you do whatever. Cure wounds could easily be interpreted as returning the body to its natural state as the soul percieves it. If wanted his legs back more than anything so much that his soul held onto it like phantom pain, then I would say maybe a Greater Restoration could if he wanted that.
But if he'd grown accoustomed to his new life and his new legs and no longer sought to "restore" anything, having made peace with his injury, then no, greater restoration would just restore him to his own healthy self image. And a spell like cure wounds would do absolute dick.
I'd love to let this play out, narrate the lack of effect of this spell, and kick this asshole from the table.
There's a book I read that takes place in Faerun where a cleric is getting tortured by ogre clerics by having his limbs broken and then they use heal spells to heal his limbs at odd angles. After he's freed, they break his limbs again, heal them in braces, but he had a permanent limp
DnD healing can only do so much before its just some high power reality changing magic.
My read of simple HP restoring magical healing, at least in D&D, is simply that it is equivalent to accelerated natural healing with no potential for complications. So if whatever ailment you're trying to heal wouldn't also be healed by any arbitrary amount of rest and recuperation then Cure Wounds won't cut it either.