He might not have been able to get a message to her, but Bryn had two things they didn't, a sense of extreme emotions, and hearing better than they did. And she was getting a very ohshit moment from Dan. She had needed a tube of the oil from her bag anyway, and a match.
When she burst through the door she was holding her oil coated dagger by the blade, between her forefinger and thumb. She launched it at Derwent's back before the old dead bastard could turn around. "What did I tell you earlier? He's mine, and I'm going to start taking offence to the fact that you keep damaging what's mine." Of course, she thought hard at Dan. (That is not why I'm helping you. There's etiquette between the dead. Or there is supposed to be.) "And I'm going to start carving it out of your asses. I don't care about what kind of deal you made Derwent. Danny wasn't part of it."