Unless he wants the aftermarket for looks? Or future overclock?
Different story.
You must have thermal paste even if you don't OC.
Yes, but only if it doesn't come pre-applied to the HSF. I have yet to see one like this, unless he's buying an OEM CPU with no HSF, and therefore no TIM.
It makes a BIG difference. It helps in taking heat away from the CPU to the heatsink/fan.
Sure it does, but really only when you are OCing, or if you are running at full load 24/7. Otherwise, the stock stuff is fine. Like I said, there's a very minimal difference.
Your friend needs to apply thermal paste/grease.
Again, only if it's not on the HSF already, which is highly unlikely.
MX-2 is good, so is Arctic Silver 5
I recommend MX-2 over Arctic Silver for any beginner, or even advanced, computer builder because AS5 requires a curing time. You'll get almost instant results with MX-2, but again, this only for OC or high load situations.
Zalmans stock paste that came with my CNPS9900 was good!!!
Like I said, minimal difference between all types of TIMs made for computer HSFs. They are all manufactured to do the same thing; buffer the heat between the CPU and the HSF in the very minute cracks of the CPU and HSF metal contact plates.