In Wesker's Report, a non-interactive promotional DVD, it is revealed that Ada was rescued at the last minute by Wesker, whom she was working with throughout the course of Resident Evil 2.
The game Umbrella Chronicles uses artwork of Ada in Ada's arms, a scene from Leon B. The book Resident Evil Archives, also summarizes the story according to the events of Claire A/Leon B. In Leon's B ending, Ada tosses Leon a rocket launcher, which is a good indication that this is indeed the true ending. Not only does it make more sense that she would aid Leon rather than Claire, but she also does this again later in Resident Evil 4.