If user admin give them to much access you may need to create your own role and then ACLs for that role. I can think of at least 3 you will need.
1. Read access to the table (you may not need this if they have itil).
2. Write access to the table (this will be needed before you limit them to just one field).
3. Write Access to the field you want them to be able to edit. This will limit them to just the one field.
