Honestly, I'd try to find a third party with a level head who can back you up in regards to etiquette and professional conduct which they've seemingly overlooked, at the least.
What they've done is likely out of naivety and ignorance (read "obliviousness", not "active-selfishness") and an ideal outcome would be for them to aknowledge they're at fault and compensate you (perhaps excessively, if we're still talking ideals!)
It doesn't feel good to fight your own fights when you're ethically and legally right though potentially can look like an arse to those who undervalue your personal skills, efforts and investments.
I feel I'd step in if I were a colleague, but I'm not... There's a lot of sensitive psychology at play here which can be difficult for the most level headed and impartial to handle with ease.
In short, you've been taken advantage of, there's at least another human being who needs (NEEDS) an education in professional (mis)conduct and, er, there's a chance you're going to be daemonised simply for being a victim of circumstances and I highly recommend seeking the level headed, passionate and likeable allies whom I recommended above
Please do keep us all updated on the situation, Barrfly - at the very least the rest of us can get an education and some perspective in regards to how spectacularly tits-up or productive this winds up being