I couldn't send him the pdf as that doesn't send the file exactly as an ePub. PDFs are always sketchy at best.
Email needs to come from my business/company one (though I have used my gmail account in a pinch).
However, I will try signing onto that account directly vs using apple mail the next time. Good idea.
What did happen is that I made a copy for Kindle preview and I also sent the epub exactly as it showed up, but I turned off everything on my end that was all about Reading, Buying, Sharing, Saving, Starting automatically, Opening the Store, etc. on Books.
He was able to open the ePub no problem (though on a PC, and dragging to an Android, and the ePub automatically showed up as "BOOKS" for the icon. he didn't say if he attempted the kindle version. So, I assume it was turning everything off in BOOKS that helped. Or, there was a glitch and this was all just wasted time, which seems to be mostly the norm these days.
In any event, I'd still like to know how to get the BOOKS icon to not be the default, even though apparently the only way to do that is purchase an Epub reader app or remove the BOOKS app (and I do use it) to trick the MAC into not assuming anytime I try to share an Epub the person naturally has an Apple product.