Sacagawea was qualified to act as a guide for Lewis and Clark because she was indigenous to the region.
Lewis and Clark had little idea as to what they were going to encounter. They did not know the land, the people, or the terrain as well as people who were indigenous to the region. In this regard, I think that anyone would have been qualified to serve as a guide for them.
Sacagawea was uniquely qualified to serve as a guide for them. She had lived her entire life in the region. The way in which her people, the Shoshone, showed love for the natural region caused her to know its intricacies, making her vital for Lewis and Clark. At the same time, Sacagawea had to become used to frequent movement in the region. The Minnetaree warriors kidnap her and her cousin, forcing them to move to a different area. She is kidnapped again and forced to hide in a different area until she is rescued. This enables her to get a firm grasp of the terrain.
Language becomes another reason why Sacagawea is uniquely equipped to help Lewis and Clark. She understands English in addition to the indigenous dialects of the different tribes. "Janey" proves to be invaluable with her assistance in this light. She is also helpful because the novel shows her husband to be unhelpful. Finally, Sacagawea is an honorable woman. This makes her qualified to help Lewis and Clark because she is selfless. For example, when the boat falls in the water, she could have seized it as an opportunity to undermine them. However, she saves the journal and compass. In these ways, Sacagawea was fit to help Lewis and Clark.