I am.
Epic levels add ab progression 1 per 2. Starting at 21 (Then again at 23, 25, 27, 29, 31, 33, 35, 37 + 39).
Epic AB will never increase your number of attacks.
So her pre-epic BAB is 15 as it is (16 BAB (at 21, shown on the sheet) - 1 (the epic BAB).
Which is 1 off getting a 4th attack.