Hey Jrod,
The only way her phone number would be linked to iMessage is if she had an iPhone in the past and signed into an Apple ID, it would link it just by signing into a phone that's not an iPhone with the email account.
Go to the link below and enter her phone number in the section that says she no longer has an iPhone and see if that works. Or provide more information about the issue, like if she owned an iPhone in the past.