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If he's not willing to go custom or bespoke then I'd agree with @amey that an off the peg Aethos is the best bet. Every review I've read states just how much 'fun' it is to ride and how much nicer it is than most modern aero bikes, and unless he races the aero penalties are inconsequential.
Saffronspokes-
I know I know, you're preaching to the converted here but he's been sipping from the marketing chalice and lapped up the aero mumbo-jumbo (in spite of not racing). So sadly that option is out.
I should add that he has said that he might be selling around 5 years' down the line and then buying whatever is around then. He seems to be the cycling industry's ideal consumer but I'd be dishonest if I didn't admit that we all have a little urge to ride the cutting edge of R&D, even if it breaks the bank for the most marginal of returns.
gdkhgd
Extra considerations: it's Belgium, not Girona and so he's eyeing up the aero categories. To me, these are some of the most over-stated, garish, and quickly out-dated contraptions on the road. Having been half-seriously dubbed "chief advisor" in his pursuit of pedal-powered nirvana, I'm trying to steer him towards something a little more refined that might even please me to ride alongside come summer. FWIW, I did offer to carry his bidons for a considerably smaller fee than 10k which would save him far more weight than what this kind of cash will shave off. I also tried to talk him into the custom steel - or at this end, titanium - route but to no avail. Fair enough, it's not my money so not my call.
I guess that besides rolling my eyes a bit at the lack of imagination in this folly, I'm wondering whether there is anything out there that ticks the boxes and is a little more interesting than just going for something seen in the pro peloton.
Of course, he doesn't have to spend 10k, but it gives an idea of how much is possible. Do you get a better deal buying something fully specced I wonder? Were it me, I'd find the frame (with this cash I'd probably splash on a one-off paint job), choose groupset and finishing kit, and go for handbuilt wheels. I guess it just shows how different people's approaches are to bikes and where some like to geek out on the minutiae, others pick something off the peg. Maybe the latter would save me hours on the forum...