I wonder if he purchased the license through a vendor which supports o365, or from Microsoft directly.

It sounds like he's the sole user on an o365 account. I wonder if his account was compromised leading to the loss of access. Seems kind of negligent on his behalf if he's the sole (admin) user. If his company has compliance requirements, meeting them would have likely addressed this issue.

Quick temporary fix would be to change public-facing DNS records: MX and likely (hopefully) SPF & DKIM, pointing them to a different mail host. At least that way he could get his email.