Unless his parents are multi-millionaires that makes him even more of a dumbass in my book.
He's taking money from his own mom and lighting it on fire just because he can.
A better person would say "no mom, $3K is all I need, spend the rest on something you want."
Hey, I don't agree with the whole situation either. I bought and built my parents their current desktop PC and my dad still tells me it's running very smoothly even though it was a budget build from 2008. The last thing I need from them is to spend money on me. If it's going to be a gift that they insist on giving, then make it something that I actually care about rather than a $5,000 computer that will be worth $500 in about six months.
That said, this isn't my situation, nor does it sound like there's going to be any behavioral change coming if this guy is getting married soon and this is considered the norm to both the mom and the son.
Some of the conditions of this build are indeed absurd, but there's no reason to not get an i7-2600K, 16GB RAM, SATA III SSD, a Dell U3011 monitor, a GeForce GTX 590 (did anyone actually read the guy's requirements?), and some good headphones and an amplifier. If he's tone deaf, then don't spend as much on the audio. The rest of the core components really don't matter that much. After that, just buy some fancy gaming input devices or a preemptive "I'm sorry my son's going to ignore you for the next __ days/weeks/months while he plays on his $
4,000 computer" gift for the future wife.
If diverting the money to the wife is out of the question, then the best piece of hardware to get to make sure the wife doesn't get neglected would be an OCZ Vertex 3 SSD.