Or by her participating that she is knowingly involving herself in a scam. Which, yeah, it's just books - but it's pretty obviously a pyramid.
No shame if you don't see how it's a scam, the cozy blanket and glass of wine are meant to throw you, and they chose 36 because it's a confusing enough number where you don't think too much about how it grows.
She gives one book to her upline. She then sends out post to 36 more people to give her 36 books. Each one of them then needs to find 36 people each, which is now 1296 people in that level if they each want 36 books. Thus the exponential pyramid. Of course there is zero way each of them will find that many people, let alone the levels below that. It's a scam that benefits those higher up, and the ones lower will likely not receive anything.
Of course she sees nothing wrong with that. She said "Sometimes I get books, sometimes I don't, that's just part of the game". Which... it's not a game when it's real money being passed around.
On top of that, whenever we see a pyramid scheme we should be stamping it out - hard. Folks, please spot the signs and point them out. Don't be afraid to comment on posts calling them out as scams.
Edit: To be clear the idea of a growing book exchange isn't a bad one, as explained in the comments though the way to make it not a scam is to make it 1:1. You either send a book and receive a book, or if they like the 36 number, you change it to "I'll send a book to whoever sends me a book!". Then it's a true book exchange.
While the authenticity of this is dubious, I don't think OP knows what a pyramid scheme is...
Look up MLM (Multi-Level Marketing) schemes if you need a reference point.
It's a pyramid scheme that doesn't appear to involve money.
An MLM for books would be charging for the books and an enrollment fee. Typically in a pyramid scheme the buy-in is the fee and nothing ever comes back to the enrollee that isn't enrollment fees from others.
In this case the books are the enrollment fee.
Just because the stakes are low doesn't mean it's not a pyramid scheme. You buy in, and then you recruit others to buy in by paying you. Pyramid.
Now, if it was a ping that said "send me a book and I'll send you one back!" That would be a fun way to do a book exchange. But it's not, there's no reciprocation.
If you need any proof that pyramid schemes will never go away, just look at the number of downvotes.
I know, it's disheartening, but still. I know I'm personally more compromised then most. My mother lost everything in Mary Kay, an MLM - and I mean everything. Lost the house, car was repo'd, all on the promise that she kept getting that if she hustled harder she'd be a millionaire. She now works at Walmart working paycheck to paycheck in her 70s.
Because of that I'll call out scams like this whenever I can. People are dumb, and gullible, and don't see that if it's too good to be true it probably is. There's 2 big types of dissenters I've seen in this thread. First group don't think it's a big deal (and in my point of view it means you don't care as long as you get something out of it, just don't think about the people who lost below you. Second group are the ones don't see how it's a scam at all. Both of those groups are prime MLM targets, and while the stakes are low here it's important to know the signs so you can see it when the stakes are high.