I reckon there is about 1lb to save, but it would involve silly money, including Easton Haven Carbon wheels.
He’s gonna be wholeheartedly disappointed unless he reigns his needs/wants/desires in a little bit more in line with reality I’m afraid!
First off, how much does he use the dropping facility on the i900 post? Even a Thomson will save about 300g off the weight of that for starters. New Easton Haven bars would save another 50g (but this is getting stupid to be fair!), and then like you say the Carbon rimmed Haven wheels (which is asking for trouble IMO) will save a bit more again.
Now, bear in mind that skill is far more important than the bike you ride, and how much it weighs, and though he is only light (I weighed 60kg too once upon a time not so very long ago), 27.3lb for a bike with 140mm travel both ends and a dropper seatpost is quite frankly bloody remarkable! Most people ride hardtails that weigh more…
And honestly, if you rode a 26lb bike back to back with a 27lb bike, would you notice it? If that extra pound was entirely in the tyres, fair enough, but spread evenly over a number of key components I’d challenge anyone to actually be able to tell the difference without putting it on the scales… Less time obsessing, more time riding methinks, besides his 5 Spot sounds pretty much spot on for what he wants anyway…