It is really common from what I've read about/heard that people who are not properly treated for ADHD have a high potential to abuse other stimulants. This sounds like it may be the case with your friend. However, the prescription drugs that people who are diagnosed take are monitored closely by a psychiatrist and are given in the proper dose to ease the ADHD symptoms. Instead of the high given by abused stimulants, prescription medication for ADHD works in the opposite way, calming down people with ADHD in the right doses.

As for the symptoms- there is way more to ADHD than just not being able to pay attention. There can be memory problems, comorbid learning disabilities, disorganization, forgetfulness, emotional disregulations, sleep problems, etc.