Ok, I've seen some odd things in my years working in IT but this one is new! A little bit of background.
An employee, lets call them Jane, works for a company and leaves. Her mailbox is deleted but because she held an important role in the company her email address "firstname.lastname@example.org" was added as an alias to John's mailbox.
Time passes and Jane comes back to the company. A new account/mailbox is created for Jane with the same email address as she's had before i.e. Jane@company.com. The alias is removed from John's mailbox.
Now, when using OWA (and only OWA) if you type John's into the To: box when you hit enter it changes from Johns name to the email address of Jane!! To make sure this isn't an autocomplete issue I've gone into the address book (people) and selected John from the address book....and the To: field shows Janes email address.....WTH! If you right click "Janes" email address from the To field and copy/paste it into Notebook it is formatted as: "email@example.com" <firstname.lastname@example.org>
Are they cloud users or synced from AD? I would check their AD info, attributes tab. You can probably find more info via powershell, but i'm not good at it. Maybe support can handle it. They have helped me with fixing weird behaviors with addresses and mailboxes in the past.