So my friend already has a bnet account and plays Hearthstone. He wants to try WoW so I suggested RAF. He made a wow account on starter/free trial edition and is starting to level a bit, so how exactly will RAF work? Does he have to delete his current wow account and make a new one with the RAF email?

If you wanna go for raf, you can add a new wow acc to your existing bnet acc, leaving you an option to choose from your wow accs when logging in to wow.

==> you can have more than 1 wow acc on your bnet, i believe its capped at 5 or 10 per bnet acc?

My suggestion is to mainly use this new RAF account and have blizzard delete their non-RAF account if having 2 accounts bothers your friend. But your friend would have 2 accounts wow1 & wow2. There is no need to delete nor benefit for having the second account if it's still a trial account, just preference.

I have 8.. apparently that is the limit so the last time I added an account I had to have on deleted. Really not a problem though at the login screen you just select which account you want from the drop down. If you always use the same one you never have to change it.