You should not change his Adobe ID. You can in his Account add a secound Mail adress (for Passwort reset in the furture). For this should used his current Mail address. And the old Mail adress should still your Adobe ID.
Adove Digital Editions secure accounts based on email address are presuming that you will keep your email address for life.
I've crashed my computer a few years ago and forgot to keep the 8 years old email account "hardcoded" to my Adobe account.
I suppose it is a way to try to sell a digital book twice.
Not the e-mail are your need you need the access to the email-account. Conect the customer support because need access to your account. Sometimes exist a possible that you get access again.
Indeed, it looks like if a email address is not valid anymore (account deleted), login denies access and declares your password wrong which is false. It is like you do not exist anymore. This is over simplified logic, real life is never like that.
As far as I understand ADE support web page, it is not free and I do not intent to pay to get support.
So far I succeed to change my email address on one of my ebook vendor web site.
There're probably two others to do.