Hello Sadhana,
Technically speaking, the product owner is the "voice of the customer". As such, the product owner is primarily responsible for product backlog refinement in terms of " building the right thing".
From your question, you imply that there is an "internal" product owner, who is part of the scrum team, who is knowledgable of scrum practices but is separate from the client/customer who understands the requirements more intimately and fully than the "internal" product owner.
I think the appropriate answer to your question is that it depends on the situation. For example, what is the product owner's level of knowledge of requirements & priorities, risks, etc., how experienced of a product owner are they? What is the nature of the new requirement? What is the nature of the relationship between the product owner and the client/end-user?
I hope the above clarifies some items for consideration when addressing this question.
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Yazmine Darcy, MBA, PMP, PMI-ACP, CSM
Director of Operations, OSP International LLC
The Project Management PrepCast™ -
www.project-management-prepcast.com