Ah yes, I am quite familiar with this scenario, as I have recommended many friends for jobs in my industry, some who had previous qualifications and some who didn't, and without a doubt I will always be very up front with my pressure on the hiring group and those that it will effect, and with the friend seeking work as well.

If I am in the hiring position, I disclose my relationship to the candidate to all who are deciding (this happens often with freelance positions,) and be sure to objectively evaluate against others, and then only if there is an all things being equal type tie breaker needed do I press my influence to tip the scale. I have this argument with my union leadership all the time when they want me to hire previous union members first and I refuse to use that as a metric unless there is a tie breaker needed.

If I am not in the hiring position but just applying pressure to consider my friend, I am very cautious, just as open about the relationship to the candidate, and never make reciprocal promises. It is not unusual for my friend candidate to be up against other co-worker's friend candidate too so the waters can be quite muddy. I communicate my expectation to the hiring team, how I would rank the candidate for the position, especially if they are up against internal candidates that I know well. I know I will be held accountable for a poor recommendation if it goes wrong, so I take it very seriously.

The situation as you have described sounds like caution is not being exercised and your friend could end up doing more harm than good. And the current coworkers are not likely to forget being treated poorly. That is a very hard thing to watch. If the friend is more qualified, then there is really no issue that honesty can't solve, and on the other hand if the opposite is true, they are definitely screwing someone over and deserve the backlash.

You DO realize he's the one having sex with your mom. Right ?

IMO, letting the A know about the job and even putting a good word in for him isn't out of bounds in that case, but really going all out might be bad form. Putting myself in the D's shoes, I wouldn't be upset if a coworker recommended an out of work best friend for a position that I was looking at, unless perhaps they were looking at bending some rules, like if the position was up for internal employees only.

If I was the D, I wouldn't destroy the relationship if the guy's friend got the job... a lot of positions are about who you know, and just as I've used connections to help land jobs, I've probably been passed over for someone with better connections.