The weather is the only thing that could help him. Though, had Nadal been marching into the final, things would look different. It's not a given that Federer would reach the final then. He surely wouldn't be as motivated as when knowing Murray was waiting there.
And you are not even a Nadaltard, are you? You are starting to sound like one. Federer is favourite against Nadal on grass, his lousy H2H on clay and slow HC doesn't enter into it. Federer knows he is 6-1 against Nadal in the 2nd half of the season, a much better percentage than he has against Murray, so his motivation to reach the final would not be diminished by an iota of a percentage point if Nadal were there instead. You might also remember that Fed and Djoko played in SF #1 and would not at the time know who would reach the final from the other half. You
might take it for granted that Nadal would beat Murray, but I can assure you that neither Federer nor Djokovic would make the same assumption.