Heterogeneous refers to a mix of dissimilar compounds, while inhomogeneous is more about a varying of properties throughout a single compound.

So in the abstract, the friction surface is called inhomogeneous because it's referring to the floor as one component with a varying friction coefficient.

subtle difference: "heterogeneous" generally implies a mixture of distinct, separate elements or components, while "inhomogeneous" can refer to a lack of uniformity in properties within a single substance

at least in chem