Her computer, shared bill for access, traffic tapped coming out of the computer in their home at her request. I had plenty evidence enough using browser history that was kept by IE despite his efforts to clear it. No snooping software was installed. We could prove he was cheating just based on that but the network traffic allowed her to find out times and catch him red-handed with a computer he had hidden. This after she confronted him with the browser evidence and he had sworn he'd stopped. My conscience is clear, some of the crap he was doing was extreme and it impacted the kids. Risk coming home with a disease and kill her? Not cool and I was happy to have helped. In the end he confessed and none of my efforts ended up in a courtroom. I simply helped her confirm he was a dirtbag and that it was time to pull the 'chute after years of BS. Removing doubt in her mind helped a great deal.
To be clear, this wasn't using any of my own infrastructure and no one comes to me for ISP type services. What I run on my own infrastructure for friends isn't anything naughty and they're all fully aware that I can see metadata. No different than any other web server in that regard!