Imo you can still be a fan. Yes, the books have aged poorly and it's good to be aware of the odd implications of some of the lore but the most important thing should be not to give her any money over it.
The problem is that even when you pirate the stuff you are maintaining its popularity. You'd have to pirate everything, never buy merch, talk about it, participate in anything related to it, never show the pirated media to any friends, etc. What is the likelihood of that?
How? Definitions I can find are all similar to this:
Separating the art from the artist is a known concept defined as a manner to discern people’s appreciation of an artist’s work from their hate for the creator’s actions or opinions.
That is all. Consuming the art and appreciating it no matter who created it. This doesn't mean it has to be purchased. The idea is simply that you don't have to say, burn Rowling's books that you have purchased once upon a time as mentioned in the opinion article. Or delete some artist's music from your music player, put movie DVDs into microwave oven, delete all photos of a certain painting, etc..
I've seen many people at conventions associate with ghe Marauders fanfic than the actual harry potter books. Treating that as the 'canon' books, as to not give money to JK.