AFAIK, the important thing in polyamory is consent. It looks like the boyfriend didn’t consent, so that seems to be a form of cheating, though it’s possible that it’s an honest mistake - maybe it hasn’t been properly established that they have to tell each other first when they want to date someone new. Though it’s also possible that the relationship wasn’t actually open in the first place, or just not in the way “Asher” said it was.

Bit hard to say who was cheated on without knowing all the facts.