If I have an ebook on my computer in e-pub format and send it to a
friend’s computer, can they then send it to their Kindle via a USB cable?
Epub books on a Kindle
Well, the answer to the literal question that you asked is yes. They can send it to their Kindle.
The problem is the Kindle won’t be able to read it. In other words, they won’t be able to read the file. So the practical answer to your question is no. Kindle does not support epub format.
However, there is a solution. There’s a program called Calibre. It is (among many things) an ebook conversion program. It’s actually a desktop ebook reader, but it will do conversions of one format to another.
Convert to Kindle format
You can use Calibre to load up your epub book and then save it in .mobi or MOBI format.
MOBI format is actually the native format used by the Amazon Kindle. They put a DRM layer on top of that, but fundamentally, it’s the underlying format used by Kindle. In fact, the format I use to upload a Kindle book to be published is the .mobi format.
You can perform that conversion yourself using Calibre.
(Transcript lightly edited for readability.)