The observer would be able to view and participate in the review, but wouldn't be required to complete it. I.e., they could add comments and even bugs, but if they do not participate in the review at all (e.g., due to time constaints) the reviewer(s) could still end the review as normal with the observer required to interact at all. This would give everyone notification that the review has begun, and make it clear that their role was optional, so to speak.