Not sure why Ril wouldn't leave the party once they teleport back to the mansion. The condition is just to make sure that Erebus is dead. So if for some reason he isn't, Ril won't leave the party.
Sorry, English is not my first tongue so I may not have explained what happens correctly: After killing Erebus and going through the mirror (thanks to your most recent bug fix), Ril does leave the party. When he does so, he asks to come see him at his home for the reward. When I do so, he does not start any "reward" related dialog. He just start the " do you need me in your party" dialog. Sorry for the confusion.