4473 states "Illegal OR addicted." As replacement therapy for another addiction such as heroin it qualifies. Methadone is a schedule-II controlled substance. Current methadone users must answer "yes" on a 4473. You must read the actual law to see this though. It's not clear on the form. It doesn't matter if the drug is prescribed. Unlike many other schedule-II drugs used to treat pain, it's main use is to treat addiction.
You can be busted for DUI while under the influence of methadone, depending on the concentration in the body. You can be arrrested for reckless endangerment in many states while carrying a firearm under the influence of alocohol or any narcotic or opiate or controlled substance, again depending on the concentration. Even marijuana users can be charged with DUI. Hell, you can't even enter a restaurant in some states because they sell alcohol. You have to dig through the law and case-law applications to get a better answer.
When he no longer needs methadone I would welcome him to the community.