In order to meet Riordan make sure, Rylan (his brother) likes you (good terms with him).
After the fight with Sithyrian factions on the L-17 comet. (Bradley Boss Fight). Forgive Rylan (whether you jailed him or not) Then talk to Rylan on his room with the dialogue given.
After you forgive Rylan, land on Sithyria. Bradley will ask you if you would help on defeating Arihama, after that Cicero will say his temporary farewell, after that Rylan will come out of the ship and HE WILL GO TO HIS BROTHER. Choose to COME WITH HIM. SO YOU'LL SEE RIORDAN.