Good question, and I'll answer this one.
Autism Spectrum Disorders (the new official Umbrella term for Autism related disorders, including Aspergers) is, like any mental illnesses a bit tricky to diagnose. Many professionals who are trained in the field have difficulty finding the right one. My psychatrist agreed to this diagnosis based on what I relayed from what relatives told me, (Inability to correctly guess nonverbal cues, understanding someone one moment and then having that understanding leave your brain the very next, beign cronically spaced out, social clumbsiness, and other clues) and agreed to that diagnosis. Unlike your friend, I put it out in the open because it best explains the occasional mental miscue I show, both online and in real life. (I make a joke aobut it: "Sorry folks. Autism just Spoke.")
Unlike some mental illnesses, like Depression (low levels of Seratonin) or Schizophrenia or ADHD (too much Dopamine) ASD isn't treatable with medications because it' doesn't depend on the more familiar causes of mental illnesses, Neurotransmitters. (You have to excuse the five dollar words here; I've done some lay research) Autism is more of a genetic way that the brain is wired; think of it like having a Mac Brain in an Windows World, especially with the Higher Functioning Ones that make you act more like Sheldon than Rainman. Those are better dealt with by coping with the quirks in your brain, and a bit more compassion from others than what some may expect. Not that he's damaged or anything, just different.