'Decision at thunder rift,' the rest of that trilogy, or, yes, 'wolves on the border' are the best intro to the universe.
Depending on if her tastes are towards the pulp end of the spectrum, 'Close quarters' might work. 'Initiation to war,' 'heir to the dragon,' and 'wolf pack' are pretty good too. 'natural selection' isn't for everyone, but IMHO the best of Stackpole's novels.