The one who sabotaged her work based on personal pettiness.

That makes total sense. Somehow I forgot about him, and assumed it was her partner. I was like "OK, i guess they have to boot everyone who was even remotely involved until they can sort it all out and decide on next steps."

But no, your guess makes a lot more sense.

I don't think we will know unless this ends in some ugly lawsuit, but they can confirm the story by analysing multiple commit logs from same dude reverting Horvath's work.