It can be done by using the Rules module and using the user reference field in particular.When one of the fields it has is a user reference and you wish to send an email to that user of that reference each time a node is updated, try using the following piece of code:
// Change $node->field_userreference to point to the correct CCK field
$recipient = user_load($node->field_userreference);
Another option is:
Add the action Load a referenced user and select the user you want to load in the DO section. Give it a name if you want, otherwise it will default to 'Referenced User'.
Then add another action, Send mail to a user and select the name you chose in the previous step under the Arguments configuration.