I don't know if there's a link between ADHD and heart failure. That would be a question for a MD. I do know that worrying about a possible health event can increase the likelihood of it so managing the anxiety would be a good place to start.

If the MDs deem the stimulants unsafe for your friend then they probably are; I would trust their knowledge and judgment if several different MDs agree.

Has he tried a holistic practitioner? For example, acupuncture is known to calm the central nervous system which can increase relaxation while decreasing anxiety. That's just one example. There are naturopaths trained in safe supplements. Perhaps your friend could consult with a naturopath about feeling "hyper."

Has he tried guided meditation? YouTube "guided meditation for hyperactivity" or anxiety etc.

His history of chemical dependence and drug use may have led to some permanent brain damage. Even if that is the case, there are rehab professionals trained to teach adults how to adapt to cognitive deficits in order to improve daily function and improve wellness. Has your friend been had any cognitive assessments from a neuropsychologist or speech-language pathologist? If not, I would recommend those in addition to a holistic approach to physical, cognitive, and emotional health.

Best wishes to you and your friend.