I don’t believe it was a “study” of Matisse. More like a rip-off.
The difference is intent. If she admitted and was open specifically what this came from and why she did it (which by all accounts I can find she did) and there was no attempt to profit from it (which I can't find she did) then I wouldn't call it a rip-off. It would be like me paraphrasing a great statement you made and giving you credit. The intent was not to rip off your statement but to honor or study it.
If someone can show me that she purposefully did this to deceive and tried to hide the source and she attempted or did profit from it, then we can talk about it being a rip-off.