You should be able to purchase a scroll to cure it from many temples.
To the Righteous belong the fruits of violent victory. The rest of us will have to settle for warm friends, warm lovers, and a wink from a quietly supportive universe.
There are numerous spells and scrolls that can protect and cure petrification, the characters technically don't leave your party as much as they can't do anything while turned to stone.
Mages have:
Level 1
Protection From Petrification
Cast on your party members to make them immune to petrification attacks for a short time. Most spell stores stock scrolls of this.
Level 5
Spell Immunity: Alteration
Mages can protect themselves from alterative magics that alterform. Some stores sell this but I know that thief in the bridge district outside the "Derelict House" at night seems to always have this.
Level 6
Stone to Flesh
Returns a person (or animal) back to their natural form. When you've done the slavers quest in the "Copper Coronet" the Beast Master drops a scroll of this.
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As mentioned temples with sell scrolls of "Stone to Flesh" and quite possibly "Immunity Petrification" you might even find a ring that can cast stone to flesh once per day if you look hard enough.
As a final note if the statue of the person takes damage it will crumble and restoration will become impossible.
Warning: logic and sense is replaced by typos and errors after 11pmSpoiler
It's not just green cleric spells that can do stone to flesh. There's a sixth level mage spell called "Stone to Flesh" that can be scribed and cast too. Don't recall which merchants sell it off the top of my head, but I don't think it's too ridiculously rare, and I think all spells are sold in ToB. Once your mages have that, unless -all- your mages get petrified you should be okay.
EDIT: Duh, sorry, Deadalready, just realized that you already addressed the mage spell. Thought you were just talking about the green version.
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To the Righteous belong the fruits of violent victory. The rest of us will have to settle for warm friends, warm lovers, and a wink from a quietly supportive universe.