We've had some going back and forth weither a Cylon can be a Sympathizer to side with the humans to win the game. Similar to if a human gets it they side with the Cylons.
The main reason for me thinking a Cylon can side with the humans is if he's not revealed(otherwise if he was he'd give his cards away) and at sleeper phase he gets the Sympathizer card. In the core rulebook on page 19 in the first paragraph under Sympathizer it says:
…"The 'You Are a Sympathizer' card represents a human or Cylon who wishes to side with the other team"…
I would take that as a Cylon can get the Sympathizer card and side with the humans.