It was faithful to the book. A faithful adaptation for me needs to be faithful to the themes, plot, characters, and ambience of course, but never at a full 100% ratio. Often a movie that tries to be 100% identical to its source material misses the mark. Coppola's version of The Great Gatsby is a prime example for me. In this case, the character in question is a minor character whose changes do not impact the thematic structure or plot of the story. I didn't even know this was done until reading this thread and read the novel twice--albeit more than three decades ago. Chani being a stronger, more equal character, actual corrects a weakness in the original novel and reinforces the thematic structure of deconstructing religious extremism and flaws of faith and belief.
Your definition of "faithful" may be fine for you, but you're still missing a couple of important points.
First, "faithful to the source material" means not making significant changes. Turning Chani's father into a woman is not a trivial change. It directly contradicts multiple instances in the novel where Liet-Kynes' life and Chani's family ties are discussed. We know from the novel that Chani's mother is already dead. So this bizarre new character Villeneuve introduced can't possibly be Chani's mother.
Secondly, you're missing the fact that even though Dune takes place 20,000 years from now, it's still a feudal society. That means not everyone is equal. RHIP, and among the Fremen, the only way for a woman to have rank was to become a Sayyadina. Chani was not a Sayyadina, and while she and Paul were together, they were never married, either by Imperium or Fremen customs (when Paul asked Chani to look after the water rings he received for Jamis' water, Stilgar made it clear that in this instance it should not be taken as Paul offering her the rings as a pre-wedding gift).
Finally, people who characterize the novel version of Chani as weak and submissive are really missing the boat. Yes, she's upset and uncertain when Paul announces his intention to marry Irulan. But he makes a promise that it's strictly for political reasons and he has absolutely no intention of begetting any children with her, or having any sort of intimate relationship with her. Chani is the woman he loves, the woman he wants to have his children, and during the course of Dune Messiah, she is strong enough to understand and accept this. At no time does she rage!flounce or mock Stilgar (if Villeneuve had really been faithful to the novel, the point would have been raised that Chani was calling Stilgar out, challenging his fitness as Naib, which would have resulted in a fight to the death - something no Fremen woman would do, because Fremen women don't get to be Naibs).
).
