Where has his income been declared in the past? If he a French resident his income should have been declared (form 2047?) If so, it should have been taken in to account in calculating the monthly amount to be deducted.
I had a similar unfathomable error on my return last year referring to 2041. I made several attempts but couldn’t resolve it so I went to the impôts, the fonc took one look, said not to worry and ignored it! If you can’t resolve it, you can make an appointment on-line for a 30 minute appointment at your local impôt office.