The Agile Manifesto explicitly values customer collaboration. Taking an oppositional role is contrary to the spirit of the Scrum framework. The Product Owner should instead be facilitating the client's informed decisions.
Relative Roles of the Product Owner and the Stakeholders
The role of the Product Owner is to proxy the interests of the stakeholders, which in this case is the client. To the extent that he believes that the project's deliverables will not serve the interests of the stakeholders, his responsibility is to communicate with them, offer possible alternatives, and educate them about the costs and trade-offs involved with their decisions.
In the end, however, the Product Owner is there to empower the stakeholders and drive the project, not to dictate objectives to them. It is up to the client to determine whether a project's methodologies or deliverables will provide value to them.
Enable and Respect Informed Decisions
If the client is making an informed decision, then your company should respect that if it wants to continue doing business with that client. If the client is not making an informed decision, then that represents a failure on the part of your organization or methodology to effectively communicate with the client, and you should carefully re-evaluate how you engage with the client going forward.
This is an opportunity to inspect and adapt your organization's engagement practices. Collaboration and communication are essential components of any successful Scrum project, so seize this chance to improve in those areas!