If your friend is single-hearted, she's the same person to everyone; she knows what she thinks, means what she says, and puts her whole self into whatever she does. And you are lucky to know her!
Single-hearted is similar in meaning to wholehearted: The image it brings to mind is of the person's heart, or center, not being divided into parts. It's not split between conflicting loyalties or motives, not divided between thought and action, or between a secret inner self and the self shown to the world. Single-hearted people are not always second-guessing their ideas and plans — they make a decision and stick to it, set a goal and then go for it with all they've got.