She would of had to complete form CPT30, Election to stop contributing to the Canada Pension Plan, or Revocation of a Prior election. When this form is completed she had to give a copy to her employer -  then send the original to CRA- Which then the "election" chosen to either stop contributing or revocation will take effect the 1st day of the following month- that she gave the copy to your employer.

Your daughter should have a copy of the CPT30 which has the date on it. - Which would show the date she wanted to stop contributing to CPP- then one that showed (revoking) wanting to continue to pay CPP.

If you need more help- please contact CRA directly so that they can assist you- https://turbotax.community.intuit.ca/replies/2647704