If he wants to add notes that nobody else sees, he can turn on Settings > Account Sync > Personal Sync > Notes. Then he will see the existing notes field, which will be labeled Shared Notes, plus a Personal Notes field.
However, if he enters personal notes for a song, those will display instead of the shared notes when he views a song. So if you are using the shared notes and he needs to see both, he can add a custom field assigned only to himself and add that to his layout, instead of using the Personal Sync option.