I realize now that my comment may have sounded like someone trying to prove all of Horvath's claims to be false. That was sincerely not intended. I apologize for that.
Frankly, my purpose in providing a plausible defense for GitHub is that the TechCrunch story itself only presents a single side of the story, though I understand that sometimes that's necessary for someone to come forward and draw the necessary attention to an injustice like this.
It is, however, easy to read the TechCrunch article and immediately point 100% of the blame on GitHub when we don't know the facts. I think it's helpful to step back and look at the accusations for what they are: accusations.
As others have pointed out, GitHub's response says very little in terms of confirming or denying the accusations, but it does say that they are professional enough to take this seriously.
Horvath may be doing GitHub (or various people at GitHub) a favor by going light on some details - the facts may be far worse than they appear on the surface. She (or the TechCrunch writers) may also be skimping on details in order to paint GitHub with broad strokes. There are numerous plausible motives in either case, but I won't speculate as to the motives here as I've learned it doesn't help to spread false accusations.
It isn't fair to accuse Horvath of being anything less than 100% truthful, and by the same token, it isn't fair to assume everything she said is 100% truthful without GitHub coming forward with their side of the story.
Again, I apologize for my comment earlier leading to speculation where we (as the public) really have no say in the matter since we have so few facts. It's an unfortunate situation for everyone involved and I hope it can be resolved in a positive way for both parties. For me, at least some of the followup comments here have been helpful in better understanding some of the ideas involved in this story - e.g., sexism, appropriate handling of conflicts in the workplace, roles of founders and spouses, etc (in general terms, not necessarily in connection with this story).