keeps it locked and password protected when she's not around. Anybody have any suggestions?
Yes, and you've already been given them. CUT OFF HER CELLPHONE. Let her re-connect herself, using her own name and credit, and pay for it herself. DON'T PAY TO ENABLE HER AFFAIR, and DAMNED sure don't pay for something she's using to conduct it RIGHT UNDER YOUR FRIGGING NOSE!!!
Since the cellphone is the one she's flaunted right in front of you, that would be the place to start.
I know at least one keylogger will install itself on a recipient's computer if you can get them to click on an attached file. Worth a shot. If not, look for a time she leaves the laptop unprotected (there's always a way -- I'll tell you my story in a separate post, if I haven't already). Or, you could try a voice-activated recorder, Velcro'd up underneath the front seat of her car.
Look, you're making excuses for her, and you have since you posted here, Eeyore. If you want to know how to phrase it, we can help you do that (how about "Yes, you said you wanted to reconnect with friends and have a life outside of this home, but you've shown that you've used it (the internet) to have an affair, and to flaunt it right under my nose, and I've decided that I'll no longer be disrespected that way in my own home," if you'd like), but you have to WANT to do it, and see the NEED to do it, first.