After you speak with Septimus and convince him to stop spreading lies about Sha'ira, he asks if you're interested in doing a small job for him. He then tells you about Xeltan and asks you to tell him the truth to exonerate Sha'ira. While Xeltan may be spoken to before dealing with Sha'ira and Septimus, doing so will not trigger the assignment.
Septimus will give you a datapad with proof of Sha'ira's innocence; all you need to do is find Xeltan and give it to him. Xeltan can be found in the elcor embassy talking with Ambassador Calyn.
Note: If you do this assignment before returning to Sha'ira, you will receive an extra reward from her.