Once I rewatched the episode I had in mind the simple answer - she timed the evolution of Abra exactly at the start of the battle, she knew, whether with Abra's experience or knew with her psychic power he will evolve.
For the first option, it's not very far to say that trainer can know when his Pokemon will evolve in the anime, I don't remember now which episode, but in one of the episodes in the Advanced series, there was one who his Pokemon just evolved and said "I knew it is about time he will evolve", showing that trainers might know when their Pokemon is about to evolve.
Another example of that is in Blak and White series, where Ash had a rematch with Lenora, she had evolved her Lillipup to Herdier saying just for the rematch with Ash, the first match and the rematch was only 1-2 days from each other, it is clear that Lenora knew that with a little more training her Lillipup will evolve or she won't say that she evolved Lillipup just for the rematch.
But the second option she knew with her psychic power Abra will evolve during the battle even have an evidence in the episode:
We see that as soon Sabrina is "glowing blue" with her psychic power, Abra is glowing too and evolving. it might even imply she used her psychic to make it evolve (which is like Memor-X answer).