So, the enemy cast Reverse Gravity and as Character A and Character B began to fall, Character A cast a Feather Fall that included both characters. That would have them only fall 60 feet after the enemy's turn and the immediate spell casting as you mentioned. Now it is Character B's turn and he uses both a move and a standard action to gain Fly. So he is out of actions except maybe a swift, however, he is flying.
If I understand that correctly then he would be able to make a Fly check to Hover as it takes no action per the skill description:
"Action: None. A Fly check doesn't require an action; it is made as part of another action or as a reaction to a situation."
If he opted to not do so then he would continue to Feather Fall down the last 40 feet. Now when he would do this is subject to a DM's call because he began falling on the enemy's turn and should have fallen the entire 100 feet had that not been interrupted by the Feather Fall. Feather Fall says you fall 60 feet per round and technically if he had already fallen 60 feet in this round, he would have to wait until the enemy's turn to fall the last 40 feet to the ceiling.
I usually do not like to split the movement up because of something like that so I might rule that they continue to fall now on their own turn instead of waiting for the bad guy's turn. It is all happening so fast that it is really not that important except that it could make a difference in what each character did depending on when they continued to fall.
Either way, Character B could continue to Hover in place if he made a successful Fly check, otherwise he would fall.