When Admiral Hackett dispatches Commander Shepard to Cyone to investigate why the fuel depots there have gone silent, he mentions that the Commander will receive help from Riley and praises her as one of the best engineers in the Alliance.
On Cyone, she and a three-person team (including a turian, Corporal Nyrek) help Shepard bring the reactors back online. Reaper forces attack at one point, but are driven off after killing one of her soldiers. Later, towards the end of the mission, Reaper forces return and Riley informs Shepard that she's in danger of being overrun. The Commander then has the option of sending a squadmate to help; if one is sent, Riley and the rest of her team survive and express their thanks. If the Commander does not, then she will sacrifice herself to save her squad. From the Spectre Terminal on the Citadel, Shepard can also help her get a commendation for her actions on Cyone.
If the Commander saves her, then she later sends Shepard an e-mail stating her thanks once more and mentioning that the Reapers have launched more attacks on Cyone, but Riley and her team have been able to repel them.