Computer exploded! Power supply toasted?

[legendslayer]

My brothers computer exploded while i was at work, According to him, the computer without warning went dark, then a loud pop and spark followed by an awful plastic smell.
I've never had a PSU fail on me so i feel i cant accurately diagnose the situation
When plugged in, the light on the back of the PSU turns on momentarily then remains dark and the mobo light stays on indicating its receiving power but the computer wont turn on.

There was also a rather strange black residue inside the case, my brother thought the GPU was the cause of the issue since so much residue was caught in the GPU heatsink and around the bottom of the case but the heatsink fan isn't an exhaust and the PSU is located at the top of the tower so the falling residue was merely sucked into the GPU. or at least that's what i think?
Whats going on?

Also its a DELL OEM PSU, so i'm unsure if there were any fail safes in its design. whats the likelihood it took other parts with it? The mobo light still turns on so my hopes are high.

 

[Karadjgne]

Psu is fried. Cap on the 12v rail is the usual suspect as that rail sees the most abuse. The mobo light is more than likely from the standby power, which is usually the 3.3v/5v rails. Either way, there's no guarantees on anything, Dell uses several different OEM psu's, some of which are decent, some not so, but any ATX standard psu should have 3 protections to be ATX standard compliant. Overvoltage, short circuit, overcurrent. Any other protections are optional. Hopefully one or more will have protected the rest of your system. You won't know without replacing the psu and testing everything thoroughly.