Label the different accounts on iPhone mail
It's not evident in your question if you set up the different email accounts independently on his iOS devices or if they are linked in some other way –automatic forwarding for instance.  

If you set them up independently, you can check each inbox separately if navigating through the shared inbox is confusing.

On top of the "All inboxes" view you can tap on "&lt; Mailboxes" 



and then choose which account inbox (or folder) you'd like to browse.