Herminia is exactly the kind of friend that every girl wants: she's loyal, concerned about her best friend's spiritual life—and she has a car. In fact, it seems that all Herminia really does in the story is shuttle Celia back and forth to Havana in order to rescue Celia or her children from some crisis or other.
But in reality, Herminia is so much more than that. She acts as a repository of Felicia's history, without which we would have no idea what happened in her final days. In this sense, she plays a role much like that of Pilar to her grandmother Celia. In Felicia's short and tormented existence, this means a lot, especially since Felicia's daughters still don't know the whole of their mother's story. Because Herminia witnessed much of her best friend's suffering, she might be able, at a later date, to set those little girls straight.
Herminia recalls that her friend asked her to save her when they were little children. Herminia accepted the challenge back then and spends her entire life doing just that. Felicia tells her BFF that she has fulfilled her promise to her, especially in terms of her role in bringing Felicia to the orishas. Since Herminia's father is a bablawo, it's pretty clear who is responsible for bringing Felicia to the santería religion.
Though Felicia's turning to the gods did nothing to save her life, she seems to have made peace somehow with most of the difficulties she's faced up to that point. Herminia's encouragement in religious practice gave Felicia a sense of community that she'd longed for, and gave her poetic and imaginative sensibilities room to expand (had she lived). Although it is unclear how beneficial this relationship was for both women, it appears to be the only one that Felicia had outside of her family. On friendship alone, then, we rule that Herminia really did fulfill her promise to her disturbed friend.