old FSX was a 3 core use - you would think that with today's multi-core machines that games are would be optimized to use them? I've seen several PC benchmarks of games with different intel core counts and they all seemed to indicate that 8 cores was the best with diminishing returns after that.
FSX is now 2020 - would have thought they would have moved away from the 2-3 cores only thing, i recall back then it was poor code requiring ultra-fast OC'd CPUs with a half-decent GPU (at the time in 2009 he got a GTX460 and i5-2000 series and 4GB RAM @ 16000Mhz XMP plus a 1TB HDD and it taxed the system to death.
he has just bought a new TV too so won't be looking to break the bank as he invested £250 in an Xbox which has turned out to be useless to him as he doesn't ever game...he just liked Flight Simulator X and that was it - he would set autopilot from London to Newcastle and only be at the PC in time for take-off and landing l0l - in real-time hehe..
i know he needs all you suggest but i can tell you now he would say NVMe SSD? just tear out the 10-year-old 1Tb HDD out of the current machine and stuff like that, basically he cant upgrade his current machine, he needs a new one as the old mobo won't take new components.
he's been advised this: recommended is Ryzen 5_1500x , radeon RX 590, 500GB ssd, 16 GB RAM thereabouts
remember this is just 1080p @ 60Hz not 4K@120Hz which everyone is selling their houses to get a system that can pull this off on most games lol
thanks