First, try a thought exercise. Assume for the moment that the friend in question is incompetent, and go from there. It's pretty clear that his position in your hiring funnel has nothing to do with his ability or lack thereof.
So, the manager is incompetent, and, further, doesn't seem to care about his own competence. He uses political positioning to acquire and hold his position as a leech-like existence, and now he's using those same tools to provide similar protected, useless slots to his friends. Assume worst-case, again, for the moment. If this manager decides that he wants to make it as unpleasant as possible for you to do anything other than hire one of his friends, how bad can he make it? Does he really have any teeth on this one? Are you willing to accept a useless and unpleasant lump on your team just to placate him?
My guess, based on what you've written is that his ability to make his displeasure known is somewhat limited, and that you are not willing to hire useless wastes of space just in order to placate the man. That's a useful thing to know. Assuming it's true, that means that if the guy who comes in is incompetent, you know you want to ditch him.
Now, let's look at the exact opposite side. Suppose the terrible manager is a paper tiger. He's ditching candidates right and left because he wants to keep the slot open for his friend, but once the friend has been ditched, he'll accept someone else (possibly after trying this game a couple more times, and a few grumbles). Further, suppose that the friend is generally reasonably competent at the job, but, being a friend, acts as the manager's representative in your shop. How terrible would that be? To what degree can you make sure that the new guy tanks all of the senseless requests and time-wasting? If you have one member of your team who's actively pro-leech (possibly escalating for him, etc) how much of a problem is that?
My guess is that the damage that the new guy could do as "useful, competent, but friend of the leech" is relatively limited, but I don't know. The answer to that question will tell you whether or not you want to hire him if he is qualified.
The final question is how hard it would be to get rid of him if you guess wrong. If you decide that the kid's useless and needs to go, but the leech-manager values him and is willing to fight to keep him, how hard would it be for you to get rid of him once he's in place? I have no idea what the answer is on this one, but it is pertinent.
Still, assuming that you aren't willing to hire someone worthless in order to placate the manager, and that hiring someone otherwise effective would be useful even though he is friends of the manager, and you can get rid of him afterwards if it turns out you were wrong, then I'd suggest you try to give him something resembling a fair shake. Let yourself be a bit biased. (The way he got into your queue should be a black mark against him, as it suggests that he's probably political, and casts some doubt on his skills.)
Don't discard him out of hand for it, though. Realize that the position you're hiring him for involves a fair chunk of "leech-wrangler" (ie, causing the overall annoyance-to-the-shop from that manager to go down rather than up) and interview accordingly.