Helping a coworker with their older account

Hi Box users.  I am tech support for a company and have a user who needs to get into their older account to receive shared box content from a client.  We can log into their older account (which the email address is an alias for our company at the juncture) and access their profile.  When we try to add their current email address it says not a valid email address.  As the shares from the client are coming to his new email address, clicking on the link in his emails only takes him to his older box email with no sign of the new client content.  I really feel like all we need to do is change his primary box account to his current email, but again, it says it is invalid.  Please help!