The old tax office is responsible for their mistake and need to compensate your client for it (they may file an E&O claim with their insurance for their mistake).

Even if they don't want to amend, they need to pay your client the amount of the 'extra' tax your client had to pay due to the preparer's mistake.  Your client may want to insist that the old preparer reimburse them for that 'extra' tax, or file a lawsuit (probably small claims court).