Short Answer: Not displaying referrals from Ad is a problem that can be solved by setting Google Analytics tracking parameters in the destination link of your client’s advertisement. If your client is savvy enough to use Google Analytics, setting up tracking parameters shouldn’t be an issue.
Long answer: Nearly all adservers use a mechanism called a “302 redirect” to both track a click on an ad and then forward the user to the ad’s final destination. When a user clicks the ad, they briefly visit the adserver’s click URL (where the click is tracked along with other important information), and are immediately forward to the advertiser’s website. Most of the time, the end user doesn’t know there was a pit stop at the adserver.
Unfortunately, most web browsers don’t forward “referral headers” on 302 redirects. That is, the browser doesn’t report the referring website to your client’s website after being redirected. Some browsers do, but there is an excellent technical explanation for that here. The solution is to do the above — use Google Analytics tracking parameters. Knowledge of that tool will greatly help your client in optimizing other online campaigns as well.