Just be authentic in your friendship/association with an autistic person. A genuinely interested, caring, empathetic and positive individual will be recognised by the person on the spectrum.
Not all autistic persons self harm or express behaviours which are injurious to themselves or others. Many autistic individuals can be quiet souls and may not necessarily display the behaviours anticipated or commonly attributed (expected) to an autistic persons supposed repertoire of responses/behaviours.
Treat an autistic person just like any other person, but with consideration for their unique and individual needs, with dignity, acceptance and honest intention.