My co worker was telling me about a book series in which a peaceful alien species contacts earth, and tells humanity how to get to the aliens. What the aliens really need is help in a war against a barbaric alien species. The peaceful alien species is not capable of making weapons, because they never needed to, so they ask Earth to make weapons for them in exchange for alien technology.
closed as too broad by Paulie_D, Gallifreyan, Mithrandir, Chenmunka, Politank-Z Mar 28 '17 at 7:58
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As has been pointed out in comment, there are quite a few books which fit this description. Since it's a series, I'd guess that you're talking about John Ringo's Legacy of the Aldenata series, composed of "A Hymn Before Battle","Gust Front", "When the Devil Dances", and "Hell's Faire".
A galactic civilization, under threat from the voracious Posleen, approach the Earth for aid. Since the races involved are incapable of violence, they need soldiers and need them bad. They offer the fruits of alien technology in return for a fighting force. Of course there are two catches. First, the Posleen are scheduled to hit Earth in five years or so, and in the millions. Second, the dominant galactic race, while incapable of direct violence, is not exactly nice, and perfectly capable of hiring or corrupting humans to interfere with the orderly (effective) running of the war, with the aim of eliminating humanity at the same time that the Posleen are defeated.
The main thread of the narrative follows the career of "Mighty Mite" Michael O'Neal, along with his daughter, but there are any number of other interesting characters.
If you like military science fiction, it doesn't get much better than this.