The thing to know is there is a wiiiiide variety of behaviors on the spectrum, but the common thing is some kind of lack of social skill/understanding. Do you have anyone you can ask about this specific client?

I dated a guy with asperger's (outdated term basically for mild autism), and pretty much the one thing I had to make sure of was to reign in the sarcasm (and I'm from New England land of smartasses ahhh!! but it was obvious he didn't grasp it as easily as most people), "translate" emotional/irrational/spiritual sides of things into "his language," and (something hard for me) was to not take a lot of the things he said to heart.

If I'm close to someone and they're freaking out a huge thing I try to do is distract the person, but with him that didn't work so well because he would just hyperfocus on whatever and be completely unable to direct his mind elsewhere (even on something he was completely obsessed with like history). This is where I was unable to help because of my own stress intolerance (and in retrospect unintentionally manipulative/abusive behaviors).

Just don't expect any beating around the bush or goofing around from them I guess. But like I said, if you have someone close to the client or someone that referred you to them, ask them, and as you get to know the individual, you'll learn.