Eh, it depends on circumstances and the people involved. In all cases, the original couple now has a beef, so that's where we agree.

Where things get subtle is whether or not the other girlfriend here knew that the boyfriend was cheating or not. If that's established beyond doubt, then yeah, I can see how a confrontation would make sense. It's still not a good idea though; they've demonstrated a skewed moral compass already so that could go poorly and for probably little gain in the end.

Even murkier is how different people practice non-monogamy. Some folks are free to have multiple partners without them ever meeting each other face-to-face. Others prefer to interactively collaborate and vet partners instead. Even then there's all kinds of variations, agreements, limitations, and so on. Which is to say if someone is up front and vocal about being one of those situations, do you really know that's true? And, if on that trust you wind up accidentally crossing a boundary with someone, how would anyone parse out the truth?

So, yeah, probably don't start a beef with the third party just to be safe, unless you're absolutely, positively, 100% sure that's not going to blow up on you.