Possible spoilers regarding Raphael and Karlach's engine

It still baffles me how on earth the PC cannot ask Raphael to help Karlach. It's already established at that point that her engine is failing and Dammon cannot help, and that the Steel Watcher has possibly informed that the fix would be impossible due to it being an obsolete model. There's just no reason at all NOT to ask Raph to "sweeten the deal" by adding Karlach's fix to the pile. It would just make sense and make the devil's offer more enticing.