IRS did issue some 6419 letters with the incorrect amount. Sounds like your taxpayer is one of those lucky ones to receive an incorrect letter.

I suggest your client set up an IRS account to see what amount is shown.  My guess is it will match the actual amount received.    If for some reason they can't or won't set up the IRS account then, if it were me, I'd enter the actual amount received based on deposits on the 8812 where it asks for the amount from the letter.  

Hope this helps.