I've read Caliban and the Witch and agree with your friends, but it's 20 years old. In English language scholarship the discipline doesn't have big names medieval Marxist scholars that I know of. Also in Caliban and the Witch, she explicitly rejects strict Marxist analysis as well as feminist theory to do a sort of middle ground between them, so I don't really consider it Marxist (more Marx inspired).

From what I understand of the professional discipline, Marxist history has largely gone by the wayside in "the cultural turn." Historians are being trained to reject ideology, so a lot of historians will put a stink on anything explicitly Marxist unless it has a wealth of primary sources backing it. More modern topics have more sources available, especially for the lower classes. Basically if you wanna do Marxist history, it's way less resistance to do it for 19th century and on.

The exception is that Gramsci is still prevalent in medieval scholarship, and where I see Gramscian analysis flourishing is space-place theory. But that doesn't focus as explicitly on class analysis, more how lived experience filters everything (including class).

Where medieval history is breaking new ground these days is largely with incorporating other disciplines, namely archaeology and linguistics. I think there's definitely ground to break for medieval Marxists. If history as a profession is going through "the material turn" then there's gotta be ways to tie in dialectical materialism. But that's a tall order because economic history (whether Marxist or liberal) likes to have a wealth of numbers to play with, and medieval history just does not have that solid of numerical data.