If Dave holds a consequentialist ethical theory that only values his own life, then yes we are screwed.

If Dave's consequentialism is about maximizing something external to himself (like the probable state of the universe in the future, regardless of whether he is in it), then his decision has little or no weight if he is a simulation, but massive weight if he is the real Dave. So the expected value of his decision is dominated by the possibility of him being real.