I have heard and read that promotions, especially after the first level of management, are not based on merit, performance or expertise, but rather on being aligned with senior management, being more visible across the organization and other social/political reasons.
If you honestly believe that, then either you work in a horrible place, and should look for another job as soon as you can, or you completely misunderstand when people explain politics to you, and you should refrain at all costs from providing your opinion to the people who decide about promotions in your company.
Let me see if I can rewrite the sentence to match reality:
Getting promotions, especially after the first level of management, requires more than just merit, performance or expertise; the candidate must also be aligned with senior management, be visible across the organization so that others know about their merit, performance, and expertise, and have other social/political advantages.
It's a rare company that promotes the well connected idiot over the shy genius. Rather, they promote people who do well (for whatever management defines as well) and makes sure everyone knows it. Such people are good at discovering what management values (it may not be what you value) and at "tooting their own horn."
An example. It's an imperfect analogy but it may work. You are a cook in a restaurant. You know that certain steaks should not be cooked past medium. A customer orders one well done. You cook it to medium because that is what's technically right according to your training and knowledge of food. The cook beside you does something different: maybe goes along with what the customer wants, maybe goes out and talks to the customer to explain why that steak's not a good choice if you like your beef well done and suggests a different dish, maybe explains to the head chef that this plate is going to be coming back no matter how it's cooked and suggests the head chef take some action - in some way, this cook does something other than just "what I think is right." And what's more, if what that cook does worked out well, there's some bragging "Hey guys, can you believe that customer left happy in the end? I'm glad I [went and talked to them or whatever.]" And you're there next to this cook, seething and despising and "he's just friends with the chef" and "I wonder what this politics thing is anyway" and vowing that if this cook is promoted to sous-chef, you're leaving rather than work under such a person.
You have spotted such a person. You despise him. You think his assumptions and decisions are all wrong. You couldn't stand to work under him. You worry that your own manager will be blinded by friendship and promote this person. Let's say for a moment that is what will happen unless you step in. What will stepping in do for you? Will it prevent the promotion? That seems unlikely - you're convinced that the powers-that-be value personal friendship above all else. So all that will happen is that word of your attempt to stop this will reach your new boss. If ever there was a way to make things worse, this would be it.
Why not try to see if this person has a single redeeming feature? It's possible that understanding customers, or following regulatory changes, or being well connected to the sales side of the company, are actually good things. What could you do to reduce the chances that this person, as your boss, would make horrible decisions that would ruin the company? Can you provide a strength this person is missing? If so, there's a chance you can work together (and do great things) without being friends. For that to happen you will have to let go of despising and of wanting to warn people about him, and instead let management manage, then do a great job. Knowing the weaknesses of those around you, and helping the team succeed despite those weaknesses, is another form of politics. Perhaps it is one that will work for you.