Prerequisites: Max out Dev's level 1 friendship
Reward: None
Unlocks: Dev level 2 friendship, The Lost Briefcase
To trigger the first of Dev's friendship quests you first need to boost his friendship level to the max. Tracking Dev down is typically pretty easy, as he spends most of the day working at the Post Office in Town Hall. Whenever it's open you can count on him being there. You can also find Dev at his pirate-themed home in the south of Moonbury Town.
(Note that your friendship with Dev is not indicative of your friendship with Dan. Their friendship meters are different. Running into Dan and giving him Moon Cloves won't help you out with Dev at all.)
Once you've thoroughly befriended Dev a notice will go up on Moonbury's Bulletin Boards, directing you to Town Hall. Pay a visit during open hours and you'll find Xiao at work. The two of you will be disturbed by Dan, Dev's pirate alter ego, and he'll saunter off. You'll then find Dev at the park, frustrated by Dan, and he'll go back to work. This will end the quest and boost you to the second level of friendship with Dev. Continue to befriend Dev and you'll unlock the next quest in his storyline, The Lost Briefcase.