Your product owner is over stepping his bounds if he is telling you how to do your job technically, that isn't what SCRUM is about at all.
SCRUM is about freeing the developers to concentrate on development issues and empowering them take charge of determining how long things take and how to do them.
SCRUM is about collaboration, that is what Sprint planning meetings are for, to promote collaboration between all the stake holders; product owner, developers and testing.
Yes the product owner should prioritize features, what needs to be delivered first according to the customers needs, but the developers should be doing the engineering and design, not the product owner.
I don't agree that developers should be designing GUI's and workflows unless they are specifically tasked and trained to work with the customers and hash the functionality out with the customers directly. Programmer built GUI's done in a vacuum rarely meet customers needs.
SCRUM is about putting a light weight process that can be predictable and repeatable over the agile manifesto.
It makes me sad to hear stories that very good things are being perverted like this.