41000 expected to be paid in a month from an educator who gets taxed monthly

Sars billed my mother R41 000 tax to be paid in 2 months. She's an educator by profession therefore her salary is already taxed and gets an extra income from Sentinel as my late father's Pension Fund which is also taxed... She has submitted both IRP5... She requested to see how it was assessed but no one could help her... Not even the branch manager could explain to her how they arrived at that amount... Worse, the 1st half of the amount is expected to be paid by 1st of November then the 2nd amount goes of on the 30th of November... How can that be expected from an educator...