Sign up for Office 365
Learn more about Office 365
Currently, you can only federate one domain with an ADFS farm. If they want their users to login with just one domain, that'd work fine. If they want to let everyone use their mail domains to login, then they'll need a separate ADFS farm to federate each domain. I believe there's an update coming out that'll let ADFS federate multiple domains, but for the time being this limitation applies. That said, if everyone logs in with the same domain then there's no problem, they'll be able to federate easily.