It's difficult to say, because "kf" may have damaged any number of components by deciding that a lack of knowledge on the subject isn't going to stop him. That kind of bungling is fine when it's YOUR stuff you're destroying, but...
Anyway, when you say you went down to the bare essentials, that would be just the CPU, RAM, and PSU right? Then if the thing seems to turn on, stay on, and beep after POST, you know what is going on.
If you did that, and the thing still won't turn on, I'd say the motherboard is likely fried. The thing is getting an initial jolt of power, so the PSU is likely fine. Unfortunately it's not a simple matter to power up an ATX PSU on its own. You need a special loop device to short a couple of pins.
But given the apparent lack of skill, not to mention concern for his friend's stuff, the odds of "kf" destroying the motherboard seem infinitely higher than the PSU. I won't rule out the CPU, but even if the CPU were completely gone, you should still get a stable power feed.
One other thing, as I've forgotten to do it in the past. You did connect the secondary power feed cable, right? It's usually located very close to the CPU, and is on the order of about 4-pins. I've also seen motherboards that have 2x12-pin ATX rails, but those are usually boards designed for multiple graphics cards.
Without having seen exactly what "kf" did while you guys were at Dairy Queen, you're kind of left in the dark here. For all you know he was walking all around a shag carpeted room carrying various components with him.
Start with a base system of RAM, CPU, and PSU. If that doesn't work, you know it's one of four components. You also know that RAM and CPU are unlikely candidates, so you can focus in on the remaining two: motherboard and power supply.
So the story goes...I was talking to a friend's brother who said he could build a computer no problem and seemed to have the knowledge to do so...well my other friend (well call him hooty) ordered parts off of newegg and brought it to the "knowledgable friend" which we will now name him kf. Well hooty and i drove to kf's house to drop the parts off and went to dairy queen and back...to find out he was halfway done...but was having problems...seeing as he didn't know what half the pieces were for and kept trying to power it on when fully installed...and so we ended up bringing back to my house. I put the comp together within a few hours by reading all the manuals I could get my hands on. The problem now is that the power supply was working and running well at kf's house until about 30 min before we took it back but there was never any video feed.
The problem is now that I hooked it up *which im almost 100% positive it is hooked up right* and even just went back down to the bare necessities..the power supply and mobo lights and fans will spin/turn on for about 2 seconds..then shut off. It does it repeatedly even when i took the power off of the hd and everything that didn't NEED to be on for me to find out if the power would stay on.
Specs on the comp are
corsair 750w psu
ati radeon 5870 his turbo gfx card
regular 650gb hd
gigabyte p55-usb3 mother board
intel core i7-860(?) cpu
antec case (dont think that's needed but just in case.no pun intended
adata ddr3 ram..2 sticks but only used 1 at a time
like I said..symptoms are that the 4 phase led lights on the mobo go on for about 1-2 seconds along with all the fans (before i unplugged them all)...and the power supply fan turns on as well.
My thoughts and from what I read said that the psu was underpowered (which at 750w doesn't sound right to me)...that the cpu was toast..or that the heat sink fan was improperly installed (including the thermal paste)...or that the mobo has a short circuit in it somewhere (which i've checked all the stand-offs and nothing is touching it and even ran it outside the case for testing purposes) and can't find anything so any help would be appreciated.