The addition of a small quantity of flexible fibers in a granular material is an efficient technique to increase the yield stress of the material. While the influence of fiber addition on the mechanical strength of granular media has been studied, much less is known about the flow properties of grain-fiber mixtures. In this article, we explore the effect of flexible fibers on the flow behavior of grain-fiber mixtures above the yield stress. We use a vane geometry to study the rheology of a dry granular material mixed with flexible fibers with different volume fractions and properties. The vane is immersed in the material, and the granular pressure increases with the depth of immersion. When the vane begins rotating, we observe a transient regime, which depends on the number of blades and is associated with the mobilization of material between the blades. Following this transient phase, a stationary regime is reached. By measuring and modeling the stationary flow that develops around the vane, we deduce the effective friction coefficient of the material from the torque measured on the vane. Following this approach, we investigate the effect of the fiber volume fraction and the aspect ratio on the effective friction coefficient of the grain-fiber mixture. Our results show that the effective friction coefficient increases linearly with the fiber volume fraction and exponentially with a fiber aspect ratio. These findings provide new fundamental insights into the flow properties of grain-fiber mixtures.