It pains me to hear struggles like these; I sympathize with them since I have dealt with similar, even more severe circumstances.
To this Scrum Master, I say this: consider what your role as Scrum Master dictates for your actions in this matter. This Scrum Master will have to consider his service to the Product Owner, the Development Team, and the Organization as a whole.
The Product Owner
In service to the Product owner, find an empiric method for demonstrating that his requests run counter to his intentions. Think critically about what is going to make him look good to his boss, and help him see it is in his best interest to act accordingly. In this way, you bear his armor and may well establish a lasting trust with him. I would that counsel that Scrum Master to resist the urge to condemn this Product Owner for his assertions, but rather double down on your efforts to understand what is driving him to act in this way.
Believe me when I say, I know this is hard. There are many motivations that go beyond healthy self-determination. You may well find an irrational behavior that resists all efforts to be reoriented and improved. Fortunately, the the same analysis will aid you in making the arguments to those outside the team. You may be able to show to others in management that his efforts are running counter to the interests of the business. You needs must think like a surgeon; determine how far the infected thinking has spread. Determine what thinking represents “healthy tissue” and which mindsets may be helped to see the benefit of your improvements. Do this by establishing a steady, fair-minded empiricism.
The problem herein of course is politics, or individuals and interactions. The best advice I can give you in these matters is in how you orient your own attitude. My advice comes from the New Testament, the book of 1 Peter chapter 3:8,9
8 Finally, all of you, have unity of mind, sympathy, brotherly love, a tender heart, and a humble mind. 9 Do not repay evil for evil or reviling for reviling, but on the contrary, bless, for to this you were called, that you may obtain a blessing.
The Development Team
In service to the team, use this opportunity to coach them in self-organization; if they determine that the best way to “on-board” this new developer is to have he/she pair constantly with an established developer, the established developer may be the one who is *cough* assigned *cough* the work. This could be a struggle around which the team rallies to cement their relationships with one another and build their aptitude and inclination to tackle their own problems.
In service to the organization, pursue with honor establishing and teaching right relations and practices between the PO and the Dev team members. You know this change will increase the productivity of the Development Team; your efforts and the efforts of the team will improve and increase productivity, happiness, and self-organization. Should they become more widely accepted through the company, this effect will be multiplied to the benefit of all concerned, even your Product Owner.
Take courage, my fellow Scrum Master. Servant leadership is difficult and draining at times. In these situations, your motivation must be a well that is replenished by a source outside your team and your organization even. Find and cherish those who hear what you say and accept it, your Yes List. For my part, I’ll do what I can to encourage you along your way.