The best way to befriend an autistic is to try to be kind to their "culture" and view an interaction with them as something like a diplomatic exchange. Most autistics have unique and valuable insight, skills and abilities that can make your world a better place. (Most of the technology you use everyday was brought to the world by autistic types.) The trick is to respect your autistic friend for what they are, respect what they can contribute, and do your best to ignore their bizarre (and potentially offensive) behavior.
My name is Jason Nemrow and I am (probably) an autistic person. My oldest son was diagnosed with autism at the age of three and my oldest daughter was diagnosed with Asperger's syndrome (a variant of autism) at the age of eighteen. Although some of my other children and I myself are likely on the autism spectrum, we leave the pursuit of a clinical diagnosis up to each person as this can affect their future possibilities, especially in a very close-minded society. I personally do not seek diagnosis because it may affect my ability to provide an income for my family and because my wife has asked me not to pursue such things.
I have come up with a less-clinical and more positive term for an autistic person and persons that have some of their behaviors: Xenos. In this context, I call these folks "Xenos" people, though the proper plural form of Xenos is actually Xenoi.
My wife is not autistic, along with the vast majority of people, who I often refer to as "local" people.
A culture of Xenos is a strange beast, just because it is considered unacceptable to most "locals". Living among locals enough (and how can one possibly avoid them), Xenos folk tend to work slavishly to "fit in" among the larger society, though they often have limited success. The "culture" ends up being the experience of Xenos who fail to look somewhat "local".
Many Xenos place greater value on things besides people and relationships. This can be one of the most offensive things about them in the eyes of others. Although they can be very articulate and technically intelligent, they often are unable to understand (or care much about) how others are feeling, which seems very incongruous to locals, who seem to find emotional intelligence the highest and most desirable virtue. Xenos folks are usually perceived by others as insufferable "know-it-alls" (because they often do know much more than others in their interest areas) and unthinking elitist snobs (because they cannot take account of other's feelings).
Personally, I feel very awkward trying to address people's emotional needs and end up only attempting to attend to these needs in my children and my wife, who can attest to the fact that I often misinterpret and over-react to their emotional communication. Although the great value I place on ethics and "truth" seem laudable, my zeal often offends or upsets locals, who may value the tact with which one describes (quasi-)truth far more highly than truth itself. Most Xenos (myself included) do an extremely poor job of being tactful.
The most readily observable trait of Xenos people is their obsessiveness, often in seemingly narrow and perhaps unimportant interests. This obsessiveness is both the greatest strength and among the most therapized "deficiency" of Xenos.
Most "cures" or "treatments" for autism spectrum "disorders" are considered successful if the "sufferer" no longer displays obsessive behavior. Sadly, while all the "locals" and therapists are toasting their accomplishment, the "cured" Xenos lives the twisted half-life of a poorly-casted actor on a never-ending reality show. Personally, if I have to be "normal" for too long, I start fraying at the edges and get really snippy at everyone. I end up looking like a jerk, more of one that I would have normally appeared to be having just been left alone.