It's not really so much friction as collision, which in post-classical terms is objects coming close enough together for (mainly electromagnetic) quantum intera

It's not really so much friction as collision, which in post-classical terms is objects coming close enough together for (mainly electromagnetic) quantum interactions to dominate. The objects around a black hole are on different intersecting geodesics; "collisions" result in energy exchanges that boost the particles onto different geodesics, typically that wind closer to the horizon. Near the horizon inverse Compton scattering becomes important, too: high energy photons emitted by matter-matter collisions end up colliding with charged massive particles with a transfer of momentum from the latter to the photon. The latter's change of momentum leaves it on a geodesic still closer to the horizon, while the photon is boosted to high energy (perhaps escaping to infinity as an X-Ray).