What is the certainty?? there has been many reports on employees letting go their employers for testing positive???
ADD/ADHD is considered to be a disability under the ADA (Americans with Disabilities Act), so no, your friend who is a law enforcement official cannot be fired for his condition.
If the amphetamines he takes to control the symptoms of his ADD/ADHD are prescribed by a doctor and he can provide proof of that, then no, they cannot fire him if the amphetamines show up in a routine urine/hair drug test.