The problem Almost There has been having, I'm sure, is that she is using an e=Reader specific device that has its own proprietary software for reading books. And Kindle and Sony were originally competitors.
As far as I know, any android or Apple device can use Kindle. I use it on my iPad instead of my laptop. I can get up on my "perch" in my van an easily read on the iPad without the bulk of the laptop. It saves my laptop battery, too.
Once you download the Kindle app on your device (and Amazon will guide you to the right download), then you go "shopping" in Amazon's book section -- you can do a book section search for any type of book with the word "free" such as "Free Kindle mystery."
Be careful when looking at the list -- Amazon has a habit of letting books that are free with Kindle Unlimited service creep in among the truly free books. Just be sure you look at the Kindle version price and that it is zero.
When you've "bought" your free book (it will show like a sale but with a price of $0.00), you will get a screen about downloading it to your Amazon Cloud or another device you list. Just turn on the device you have the Kindle app on, open the kindle app and find "Archive." The Kindle app will reach out and pull in all the books you've bought. All this must be done online, of course. But once your ebook is downloaded to the device, you can access it with no internet connection. It lives on your hard drive or device memory.
Btw, as far as I've ever known, only ebooks are ever free. Paper books cost money to print so an author is not going to spend money to give readers a free paperback (not to mention shipping). I will likely put my first novel on the FREE list next week or next Sunday. Since I have two sequels, I do this to gain more new readers (which is why most authors list their books for free). Unlike many authors, however, my novels are full-length books that can stand alone if you don't want to read the whole series. Just FYI. LOL I'll post it here when I figure out my best days to do a free promotion.