Cannot add user with same domain to Power BI Pro

My company has Pro licenses. We need to be able to add a user whose email uses the same domain as the PowerBI account, and who has but who does not use Office 365. A number of other employees' emails pop up and I can add them, but I can't add this one particular user.

Does the user have to use Office 365 to use a Power BI Pro license? Can I just manually add this employee's email to my tenant since his email domain is the same as the domain tied to the Power BI account? Or does this employee have to be using Office 365 first?