My ex was "no James Bond" as mrs tribbiani said!
Didn't lock his phone, didn't log out of email (and didn't change the password which I knew), suddenly started "working late" when previously a clock watcher etc
I realised quite early on but was well advised to keep my cards close to my chest and give him enough rope - which he did!
Things reached a crunch point and I told him what I already knew/had seen and he STILL denied all...even when he told me less than 2 weeks later she was pregnant! Apparently she'd only just got pregnant as they'd only just got together blah blah blah!
Re pp they're still together...he still cheats! Inc propositioning me the day before their wedding!
That was my little revenge, except I was able to exact it without doing a thing! He knew I'd read it, it was unambiguous inc that he meant it for me (pet name) and I didn't even reply BUT he knew I had the message and could have dropped that bomb that day, I have on good authority that he basically spent their wedding day shitting himself in case I dropped him in it! 
He's not allowed to socialise alone, or even have his own sm pages etc as she doesn't trust him, he still manages to cheat.
No longer my monkeys! Her problem now.