Other things to try....
Create a new style with no parent:
- Styles & Templates > Style Manager > Add New Style
- Parent Style: No Parent Style
- Title: Default vBulletin
- Allow User Selection: Yes
- Save
Tell him to browse the site using that totally default vbulletin style and see if he still has the problem.
Do you have a test site that is a copy of your live site? Have him login to the test site and see if he still has the same problem. If so, disable all the modifications on the test site and see if he still has the same problem.
Create a test account. Have him login using the test account. Does he have the problem then?