Use FTP to rename the Thesis theme folder and then WP will use the default theme and you will be able to get back into admin.
If he edited the theme file via the theme editor in WP admin, then replace the file he edited in the Thesis folder from a fresh copy and try and reactivate the theme.
If that doesn’t work and the added code was saved to the Thesis theme options, use a plugin called Clean Options to clear out the old Thesis theme options and start over: http://wordpress.org/extend/plugins/clean-options/