click/referrals tracking plugins

My friend wants to put some ads to his partners’ websites on his site and get commission for referrals. I looked into plugins (linker, pretty links lite) that can track clicks on his end, but how will his parters know, that the traffic came from his site? I’m looking for something not overly complicated. He’s going to have about 20 ads or so (not changing).
I’d appreciate any suggestions ??