HR just may not know what it's talking about. But let's imagine for a moment that they are prepared to not hire, or fire, him because he tests positive for a prescription medication. There are two court cases I know of that have dealt with this.

First, there are a couple of questions here. Is the test itself legal for the employer to perform? If the test itself is legal is prohibiting employment based on legal prescriptions (when there is no safety issue) a violation of the ADA? The ADA is silent on the specific issue of prescription drugs in this context.

Back in 2008, the 7th Circuit issued a ruling in Connolly v. First Personal Bank that

Where a pre-employment drug test yields results that indicate use of a drug that may be legally prescribed and the employer takes action based upon the drug test results, the ADA may protect an applicant from discrimination because employers may not discriminate against individuals based upon medical examinations. [Emphasis mine]

Last summer in Bates v. Dura Automotive Sys. [PDF] (which came from this situation in 2010) the 6th Circuit seemed to think it was a close call, and the issue is heading to a jury:

Whether the ADA permits an employer to test employees for prescription medications will hinge on whether the test is a “medical examination.” If the test is a “medical examination,” then the ADA only permits it during employment if the test is “job-related and consistent with business necessity.” According to the Court, whether the prescription-drug screen is a “medical examination” will hinge on whether the test “is designed to reveal an impairment or physical or mental health,” which examines both the employer’s reasons in using the test and the test’s typical uses and purposes.

I can't find if California's 9th Circuit has ruled on this. If the friend is not hired due to testing positive for prescription drugs, he might have a case. But it might not be decided any time soon.