That should be entered your clients virtualhost entry, otherwise the changes would be global.
Another solution is to use .htaccess file to control this ( But I am not sure whether this works or not. Check it out in any case )
Code:
php_flag safe_mode 1
If this doesn't work, then adding the following lines
Code:
php_admin_flag safe_mode on
in your clients virtualhost entry would be enough.
BTW, have you confirmed that php safe mode for your client is indeed off?
Just setup one phpinfo page for your client and check if its indeed off.