There's a lot in there. Marx' brain was filled to the brim. He's not like a modern academic with one specialty. A background in economics might prepare you for the economics parts. But it's a critique of political economy and he's a great storyteller so there are loads of literary references and historical parts.

These aspects might be lost on a 'mathematical economist'. The book will challenge a lot of what an economist thinks they know; that presents it's own difficulties of cognitive dissonance.

A background in literature or history will prepare someone well for those broader narrative elements but perhaps not the equations. (I'm not a mathematician but Marx holds your hand through the equations so I understood his argument and my maths ability has shot up to the extent that I can now read and understand other economic and political economic texts.)

A background in liberal theory made Capital a fairly straightforward work for me. I started with some chapters in Part Eight on Primitive Accumulation. These are really juicy. Then I read the chapter on the working day. Equally juicy. Then I wanted/needed to know how it all fit together and how someone could have such insights. That's when I started from the beginning (including the prefaces, etc) and read to the back cover.

David Harvey talks of teaching Capital as an elective to CUNY students from all different disciplines. He was surprised to find how differently it was read by people with different expertise. The literature/creative writing students got loads out of Marx' metaphors of vampires, for example.

I guess what I'm saying is that there's something in there for everyone. Really it's a text for the whole family.

One thing that a reader will need to get accustomed to is the historical materialist method. As it sees everything as interconnected, historical materialists can sometimes flood you with information that doesn't seem to be connected. It's necessary because himat is a 'many-sided' approach.

You don't always see how all the sides connect together until the end of a chapter/book. It's frustrating at first but some writers are better at building the connections as they go and, in any event, you get used to it and then wonder why everyone doesn't do it this way. For this reason, I'd suggest reading the 'Eighteenth Brumaire' first. Because it's shorter and will demonstrate the himat structure.

Otherwise, Capital was written for the working class to get stuck into. It's far more accessible and interesting than many people think. I honestly couldn't put it down. Give it a go, you and your friend, and discuss your different perspectives and what stands out to each of you.