True it does depend on where Sabo would be in relation to Ace in this case. Also while the Marines vs Whitebeard is a big deal, the Revolutionaries are more interested in taking down the World Government. Even if news of Ace's inevitable execution were to reach them(and I can't think of how it wouldn't), they have no reason to care about him.
Then again are we to assume this is the first time Sabo has heard of Ace in years? Surely they knew Ace was going to be executed well before hand. Even if the Marines are not the big picture enemies to them they had to know of the new Shichibukai and such. This is kind of a stretch to the suspension of my disbelief.