I found a 'feedback form' saved on the desktop of his laptop. (I wasn't sneaking - I had fingerprint access to the laptop, all legitimate and set up by him.)
The 'feedback form' was something she had mocked up (I assume) and which I imagine they found cute and endearing. Personally I found it grim, humiliating and cringy.
Basically he was asked to 'rate' her out of ten on things like lingerie, sexiness, technique, etc. The language was a bit more graphic than that though, with explicit mention of specific 'acts'.
The last question asked about the quality of the breakfast supplied. He'd put a comment to say something like 'Finally I got to have an actual breakfast with you' which I took as an indication that the actual sex had been going on for a bit longer than the date on the form. It was dated for a night when I was away on a friend's hen night, so I assume that was their first opportunity for a whole night together.
After I found the form I dug around in his emails (which was sneaky and not something I'd ever done before despite having access) and found out who'd sent it. I'd never heard of her, but I gleaned a fair bit of info from her emails and his replies.
I dumped him, obviously. When he asked why I said a whole list of the things I'd found out from their messages - her full name, the name of the company she worked for, what he'd bought her for her birthday, the date of their first night etc.
His response? That I had violated his trust by reading his emails. The fucking numpty gave me access to the laptop and his email account and then blamed me for finding out how he'd violated my trust because he wasn't bright enough to delete emails or use an account that I couldn't get into. Twat.