During a sprint review, the product owner identifies a required improvement for a feature's user interface (Ul) delivered during the sprint. What should the product owner do next?

In XP, the practice that any developer can change any line of code to add functionality, fix bugs, improve designs, or refactor demonstrates:

A team member has spent 5 days on a spike and the first set of experiments has not been successful. The issue is the development team member has determined a short-term rather than a long-term solution.
What should the project leader do?

Risk exposure is the:

After seeing the planned features for an upcoming release, a customer notes that a vitally important and complex one is missing. The team estimates that this feature significantly exceeds its average velocity.
How can this issue be resolved?