If you think For a New Liberty would get a scoff (which with your description, you might be right), I think The Ethics of Liberty might be more appropriate. Much more advanced.
I doubt his An Austrian Perspective on the History of Economic Thought will be of much interest, as it’s two volumes worth of…well, history of economic thought. I doubt it’s what you’re looking for, but there it is if you think he might go into it. But if you don’t think he’d read Human Action, I seriously doubt he’ll get very far in that.
Again, Man, Economy, and State is really where Rothbard covers economics and Austrian methodology and theory, but that’s just as big as Human Action, so…
As was mentioned above, Money, Bank Credit, and Economic Cycles is more great economics, but again you’re looking at 900+ pages.
Like I was saying before, if you’re really bent on getting praxeology and Austrian theory from the greats, you may just go ahead and grab parts of those treatises and send them in a smaller compilation. (If you have a PDF editor, you should be able to easily combine sections into one file for him).
I would also check out the recommendations in the One Book for Capitalism thread. If it’s history you’re after, I think How Capitalism Saved America might be what you’re looking for. Of course, some other useful history is found in another text…as can be seen in the opening story in that thread.