I couldn't find the code of the safe, i didn't understand how to do Rival and Heavenly Husband quests either
The safe code appears in the Starmaker chat.
Heavenly Husband (Unlocks Himari Scene and gets rid of debt collector):
After a good amount of Starmaker progress, some jerk will show up at the mall on Wednesday. You have to choose the correct arguments to beat him and continue. Do the same at the beach and then meet him downtown on following Wednesdays.