In the novella The Maiden Who Travels The Planet (an official publication, by the way, so it's a canonical part of the story), Aerith's spirit encounters Biggs, Wedge and Jessie in the Lifestream. They've almost completely returned to the planet when she meets them though - they've nearly finished rejoining the Lifestream, so their conscious spirits are nearly gone by the time she meets them. But talking to her brings them back together, for a brief time, and she explains the situation to them, and they finally help to defeat Meteor by lending their strength to the collective will of the Lifestream.
So, to cut it short - they did know about Sephiroth's attempt to destroy the world, and they did their part to help out. But it was after their deaths, and it was the last thing any of them did either, as the effort (and the sense of closure it provided) meant that they faded away into the Lifestream shortly afterward. |