You need to add his domain as an addon domain on your bluehost account.
Login to your cpanel (YOURdomain.com/cpanel).
At the top of the page, click Domain Manager
Click “Assign a domain to your cPanel account.”
Under “Use a domain that is not already associated with your account” enter “HISdomain.com.” Bluehost will automatically lookup the nameservers for hisdomain.com. If they point to Bluehost (ns1.bluehost.com and ns2.bluehost.com) then you’ll be able to add it.
Choose “Addon domain”
Select “Use an existing directory” and then select the directory you want hisdomain.com to point to.
Note: For an addon domain, the directory you want it to point to need to be directly in public_html.
Since you already changed the nameservers, you should immediately see it take effect once you’ve added hisdomain.com to your bluehost account.
What is an Addon domain? (https://www.bluehost.com/cgi/help/326)
Assigning an Add-on or Parked domain (https://www.bluehost.com/cgi/help/84)
Where to Upload Files for an Addon Domain (https://www.bluehost.com/cgi/help/85)
This was found here: http://www.bluehostforum.com/archive/index.php/t-22824.html