This would not be something that his Outlook would control if no rules are in place. Exchange may have the IMF feature in place. If you do, then you may be able to see the “score” that e-mail had obtained.
Article describing feature:
The easy solution would be the next time an email from this person goes to the junk mail folder to right click on it and tell Outlook to trust all emails from that person.
If emails end up in the junk mail it is because something has tripped a flag within Outlook to move the email there. It could be a bunk mail score put in by the exchange server, or another email scanning tech, of there were to many links, or embedded graphics, etc.