I'm guessing, so I may be corrected.
When a web page is shown in an iframe it's downloaded from the affiliate's domain server, so the only statistics that would be available from the iframe would be the statistics on the affiliate's server. The affiliate's web pages could incorporate a cookie which would tell him how many downloads were from each of your client's pages in the iframe.
Code downloaded to my PC will be deleted in due course.
WIN7; IE9, Firefox, Opera, Chrome and Safari for Windows; screen resolution usually 1366*768.
Also IE6 on W98 with 800*600 and IE8 on Vista 1440*900.