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I would try and recreate the notice of reassessment. Get a copy of the T4A not provided to you and enter the information. So, treat the T4A as if you were creating an adjustment for the client. You can then compare your result to the notice of reassessment. You can update the file or save as to create a copy with the updated information.
So treat this like a client brought the T4A to you:
https://profile-en.community.intuit.ca/articles/1855593-completing-a-t1-adjustment
If your result matches the CRA results then no further action is required.
You may want to inform your client about repeated failure to report penalties:
And CRA data import may be useful in the future.
Hope this helps