Following the approach of Dahmani, Guirardel and Osin, we extend the group theoretical Dehn filling theorem to show that the pre-images of infinite order subgroups have a certain structure of a free product. We then apply this result to establish the Farrell-Jones conjecture for groups hyperbolic relative to a family of residually finite subgroups satisfying the Farrell-Jones conjecture, partially recovering a result of Bartels.