Sorry for overlooking this one. It's kind of hard to know what to answer. MT claimed in her books to know the "real" teacher of Castaneda, and claimed Casteneda was a fraud. So the Castaneda fanatics hate her like no other on the planet, while at the same time she has a small following gathered from within their ranks.
In other words, a fraud preying on a fraud, using techniques and tactics much like the original fraud. In some sense, it's ironic justice. But then again, I don't know what proof your friend would accept. After all, in the other thread, you said direct evidence, like testimony from Casteneda's own inner circle, was not good enough for you.
I don't know, short of Castaneda coming back from the dead and admitting he was a fraud, what would make your friend listen.
After all, Castaneda was debunked as a fraud by Natives immediately, and by anthropologists about five years later. And then he began openly admitting he was a fraud for the entire rest of his life, throwing it in his followers' faces and treating them with more and more contempt. But castaneda cultists claim truth doesn't matter, proof doesn't matter, etc. It's a nice scam, where wishful thinking is all that matters.