I pay for the cell phone - family plan and also one of the resaons why i have access to all her records. She is clueless about the cell records that are available. it's been my saving grace to do the snooping. I'm afraid taht at soeme point she'll figure it out and go buy a new one and I would lose my ability to snoop.
Once you feel you've gathered (and saved!) enough evidence, I'd strongly suggest cutting this off, and letting her know why ("I have decided that I will no longer pay to enable you to conduct your affair.")
Plus, anything else (hair, make-up, plastic surgery, lingerie, gifts, etc.) she might be using to conduct her infidelity.