This "treble nemesis" happened to me 4 or 5 times this year. A flawless series of victories and sudenly, out of nowhere, came this defeat against this kind of oppo (one level below you, 20-30% lower quality, active but not playing the games) that ruined your treble hopes. I am sure this is a pattern that was implemented to the game. About the "friend" trick, I always reject friendship applications that come just before a match against the manager's team.