Assuming GnomeSlice wasn't referring to himself in the third person as the 'friend' who did all this:
A more direct (but boring) question would be, how do you safely remove TNT?
Arqade is a question and answer site for passionate videogamers on all platforms. It only takes a minute to sign up.Sign up to join this community
As of Beta 1.7, TNT can only be detonated by fire or redstone, meaning it is now possible to remove placed TNT like any other block.
As of Beta 1.7, TNT can only be detonated by fire, redstone, and explosions, making this answer obsolete — recover your TNT as you would any other block.
While you can't recover the TNT, you can remove it safely with water. TNT which is submerged in water when it detonates will not cause terrain damage or trigger nearby TNT. Be careful however, as it will still cause player damage.
With this much TNT, I'd personally play it safe and drop a source block on top of each block of TNT before setting any of them off. Once placed, you should be able to remove all the TNT at once (albeit one at a time) without needing to introduce any more water.
I'll leave answering the follow-up question — how to remove all that water afterward — as an exercise for the reader. ;-)
This thread over at the Minecraft forums seems to suggest that placing water next to a block of TNT will negate it, as apparently water will fill in the space occupied by the TNT once the TNT block is removed, thus 'absorbing' the explosion.
I haven't tried it myself, and I'm not sure how it'd work for the above image. :P