My wife is self employed. She makes contributions to her CPP due to the self-employment income. Can this contribution be considered a write-off on her business?

No.  These contributions are not claimed anywhere on her statement of business income and expenses.  However she is allowed a deduction for these contributions on line 222 of her return in arriving at her Net Income.  TurboTax will automatically calculate and claim these contributions in the appropriate areas of her return.  For further info, please see http://www.cra-arc.gc.ca/tx/ndvdls/tpcs/ncm-tx/rtrn/cmpltng/ddctns/lns300-350/310-eng.html