Error after updating to WordPress 4.1.1

After updating a friend’s site I get the following http://www.healing-affirmations.com/guestbook/

I’ve never used DMS guestbook before (her site was set up by someone who is no longer on the scene) so don’t know how it should be set up.

If anyone can help with this can you give step by step simple instructions!

Thank you in advance. 🙂

https://wordpress.org/plugins/dmsguestbook/