In the Cockatrice description one of its abilities is "Start a slow transformation to stone", the Medusa has a similar ability though "Turn a body part to stone with a look". After reading through them both I realized:

There is no "Stone to Flesh" spell anywhere in the book.

That was a bit shocking for someone GMing and playing D&D for 20 years like myself, isn't it? Yeah, quite a bit. Then I closed the book and let that rest a bit, it was a little while before I finally found the answer: Fiction Dictates.

The Medusa is a mythological creature, in quite a few retelling of the tale her victims were turned back into flesh after she was killed. That would be one way to go about it right there, simple, quick and solved alongside the conflict.

You could also say that a stone to flesh spell does exist but, given how rare it is used, it is not part of a spellcaster's usual repertoire but it is not hard to find or cast, a trip to a city with a little research and it is covered.

And at last you could say it is hard to undo such an unnatural power, it is a ritual requiring hard to find and rare ingredients, making the gathering and casting of it an adventure by itself.

What would the right answer be? That is where I go back to the most common advice here: whatever the fiction states.

The group is in the middle of something more important and the enemy's ability to turn into stone was an afterthought, then killing the beast should suffice

You want them to fear the enemy but not halt the game's current direction or force the player to play a new character right now? Then make it a simple spell but one that they do not have yet, a simple trip back and you are done.

You want the game to get a new front that gives them something they want (to turn the character back into flesh) with a cost (to let whatever was happening behind and having to deal with the consequences) then it can be an entire ritual, maybe one that hasn't even been done yet. This choice seems the most interesting for a well placed opponent and even uses one of the GM moves (present an opportunity at a cost).

In the end I was very happy to have been able to break the rules mentality, I finally understood what fiction first meant and am ready to try and enforce that mindset in a live game. :)

Just want to point out that there is, in fact, a built-in "Stone to Flesh" spell: it's the Ritual move.

The beautiful thing about Ritual is that it covers pretty much everything that would be covered by "bigger" spells in D&D. The spells in DW are the ones that a wizard would need right away, so any spell that boils down to "I want to make <effect> happen" falls under Ritual.

When I was writing the Crescent Isle section, I tried to come up with a "water breathing" spell. Should it be a Wizard spell or a Cleric one? What level? Then I realized that you can just cover that with the Ritual move, since the only real effect of a water-breathing spell is "you can breath water".

My point is that while I agree it's good to get out of the "rules mentality", DW has enough mechanical weight that it's a foundation, not a straightjacket.

There was another one besides Beyond the Devil's Reach? I didn't see that on your website; it is still available?