You can already remove a number from iMessage (because this exact problem came up back when iMessage first came out)
I was under the impression it’s [1] possible but I’m away from my Mac to check.
Worse case one could change one’s Apple ID’s email.
Apple should make this stuff more obvious though
I think it’s best with Apple to log in with an iCloud email address even if you don’t use it.
It's the worse case then, and she'd have to change/remove her email from her Apple ID [1].
Apple should do better and make it easier in this kind of case. They already allow one to deregister a phone number from iMessage via a web page, how hard could it be for them to do the same for an email...