Moss helping Roy get over his ex-girlfriend through Dungeons and Dragons in Jen the Fredo.
Face it, everybody was on the verge of crying on the last scenes while playing.
Another moment that shows Moss' devotion to being Roy's friend is when he urges Roy to go to court in Something Happened.
To follow on with Something Happened, at the end of Roy's defendant speech, Moss starts a round of applause in court to which Roy mouths "Thank you" and Moss replies with a mouthed "You're welcome". If that doesn't cement how close they are, nothing does.
The fact that everyone besides Jen was incredibly supportive and understanding of the emotional trauma Roy suffered from what would normally be called a hilarious anecdote is incredibly uplifting, along with hilarious due to how they all treat it as Serious Business.
When Roy first tells Moss, he's understandably embarrassed (Truth in Television to anyone who's been sexually harassed) and begs Moss not to laugh when he tells him what happened. Not only does Moss not laugh, he sounds genuinely offended that anyone would expect him to.
Jen choosing to stay at Reynholm with Moss and Roy to make sure they're well taken care of, turning down a better job in the process.
Roy coming to help Jen when her date Daniel goes ballistic on a clown in a crappy restaurant. Even if it left him with a bloody nose.
Pretty much any act of kindness towards Richmond, starting with Jen having tea with him.