  Recently Pieplow and Henkel (PH) (NJP \textbf{15} (2013) 023027) presented a
new fully covariant theory of the Casimir friction force acting on small
neutral particle moving parallel to flat surface. We compare results of this
theory with results which follow from a fully relativistic theory of friction
in plate-plate configurations in the limit when one plate is considered as
sufficiently rarefied. We show that there is the agreement between these
theories.
