I am a forum virgin. Please be gentle.
I'm trying to help a friend who has been using the Atahualpa theme for a couple of years and has had a major problem when updating to the latest version of WordPress yesterday.
The site is www.designcare.co.uk and his home page now has the following error message:
Fatal error: Call to undefined function wp() in /home/designca/public_html/wp-blog-header.php on line 14
When trying to login to his Dashboard, this is the error message:
Fatal error: Call to undefined function get_option() in /home/designca/public_html/wp-admin/admin.php on line 21
I have had a look at his hosting account and can see his files all appear to be there, but I don't really know how to help him beyond that. He didn't take a backup of his site, unfortunately.

Help/advice would be really appreciated.
Thanks in advance.
Ian D