How to prevent mail app from marking e-mails from VIP as spam
First, make sure this person is in your Contacts, including the email address you want to mark as not being spam.  This is important because Mail checks there when determining what's spam.

Then, go to the Mail → Preferences, ‘Junk Mail’ tab, and make sure the following options are checked:


Enable Junk Mail Filtering
Choose ‘Mark as junk mail, but leave it in my Inbox’
The Following Types of Messages are exempt from Junk Mail Filtering:


Sender of message is in my Contacts
Sender of message is in my Previous Recipients
Message is addressed using my full name
Also select Trust junk mail headers in messages



After you do all this, Mail might still put some email in Spam/Junk.  Just right-click on the email and select ‘Not junk mail’.  It's not perfect but eventually it should learn.