Sarah remembered how they went with Vivian to see Gabriel in the hospital after his accident, and they ended up seeing him with Victoria. Sarah was a good friend with Vivian at that time and was furious that he kissed Vivian while having Victoria as his girlfriend.
Sarah always hated cheaters, but she can't help and wonder if she would react in the same way if she didn't consider Vivian as her best friend at that time.
One thing is certain, and that is if she knew how much Vivian hated her, she would definitely not go to the hospital to visit Gabriel, and she would not know that Victoria even exists.
Sarah has to admit that she does not know the details of the circumstances either. She took Vivian's story as-is and seeing how Gabriel is looking at them without any guilt while holding Victoria, Sarah wonders if there is more to the story than what Vivian told her. After all, it would not be the first time that Vivian deceived her.