Can't completely answer this but I had Sebille in my party, however the dialogue didn't trigger with Roost. So I ended up fighting with him and killing him but afterwards Sebille acted like I talked to him.
Did you try talking to the other one after the battle? Normally there should be a new conversation option to talk about what happened. I could talk to Sebille after the battle and she acted like she talked with Roost.
Perhaps you can also try letting them talk with Roost's spirit.
