Can client consent fix a contingent fee problem for a CPA?

Not by itself. Client consent can be important when a conflict is manageable through disclosure and informed agreement. But if a professional rule restricts the fee arrangement itself, consent does not cure the problem.

For example, a client might gladly agree to pay a percentage of a refund because there is no upfront cost. That does not answer whether the CPA may ethically prepare the return under that fee formula. The CPA still has to apply state board rules, professional responsibility standards, and any IRS practice rules that apply.