I think that says more about your friend than it does the book. When I read the book back in high school I felt it was less, how can I manipulate people, and more, how can I become someone people enjoy being around.
Just because someone's an asshole doesn't mean they're autistic.
I've read the book and had a visceral reaction to it personally. It is called "How to win friends and influence people" after all. Seems to be a love it or hate it type of thing.
I can see that kind of reaction. When I read it, the book seemed to be a lot about giving people what they want by changing things about yourself and your mindset. For some people that's a good idea (if you're a selfish jerk), and for some people that's a terrible idea (if you already give in too much to other people).
As someone who moved to the US at a young age, reading How to Make Friends and Influence People explained a lot of the apparent insincerity/fakeness that bothered me in both business and personal interactions, and that had been very much a mystery to me.
How did that go again?
Dear $FIRSTNAME $LASTNAME,
I am writing you to emphasise how much your business personally means to me and how $EMOTIONAL_STATE I feel about it...
Also remember that he was a huckster, for example changing his name so people would associate him with the steel Carnegies (no relation).
Is your friend on the spectrum by any chance?