There is very good money to be made in selling courses to do something, especially if that something is promising to change your life or make you money, or even better make you a life changing amount of money.
I don't know what kind of books she's promoting, but I can tell you I have a few books on Amazon KDP, the semi-autobigraphical diary style book that I spent 2 years writing has been out 6 years and hasn't made £200 yet despite having only 5* reviews. I put zero work into promoting it though.
The low content books I've done of colouring pages, puzzles, notebooks etc do sell, but you need some sort of graphics knowledge and it's a very flooded market now, but those take me less than a day per book to produce, again no promotion at all and from the 12 of them I make about £40-50 a month so again not a millionare but a nice bonus in the background.
If you do have graphics ability (to use canva, I'm not talking advanced photoshop) I would recommend the PoD route instead. I make around £100 a day from Amazon Merch, £20 from Redbubble (pm me if you want to sign up here as there are referal offers), a little less from some other PoD sites I'm on. Again zero promotions apart from I put pins on pinterest, I consider this my hobby alongside my main business, I enjoy design and that it makes me money is a bonus not something I rely on, but I tend to do a few designs when I have some down time just because I enjoy it. I can recommend Shimmy Morris, Ryan Hogue, Wholesale Ted, Thomas Garetz and Detour Shirts videos for learning how to do this, but I'm sure there are lots of other excellent youtubers out there. Most of them will also cover other ways of making money such as no show youtube videos, course creation, printables or templates are a big one at the moment or even just getting money from making posts on Pinterest.
You can also read more about a variety of extra income earners on makemoneywithoutajob.com if you'd like a general overview. There certainly are plenty of legit ways to make money on the side or to replace a traditional income, but you don't need to pay £2500 to do it.