I have a friend who is a disillusioned fiscal conservative... He is looking for a book that neutrally spells out the differences in economic theory between the Democrats and Republicans...

I have a feeling it isn't such a binary topic!

Anyone have any suggestions??

I don't know that the splits in economics align that well with Democrat vs Republican, especially since they are both morphing pretty quickly in their economic views.

Maybe a book on Keynes vs Hayek would be the best approximation. I don't have a specific recommendation but I know there are several books covering that debate.

The problem with Keynes vs. Hayek kinds of things is that they are meta. And that kind of quasi-philosophical thinking often doesn't help people understand or think carefully about how "partisan political" attitudes sometimes do, but often do not align with how most economists understand the social world.