Quote XoceroX (

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Is SpaceEngine well threaded? (an FX-8350 would be a very good bang-for-the-buck option if yes, or very bad if no)
Yes and no. I don't know much about it, but I do know that some things (generation of stars and galaxies, for example) run in threads quite well and will use however many cores you have, while some other things don't.
Quote XoceroX (

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How important is the GPU in this game comparatively to the CPU?
Very important. A good GPU should be the main focus of the build, my general advice is to get the best Nvidia card he can afford (AMD has too many weird issues with drivers). VRAM is the most important thing. Ideally he needs to get something with 4 GB.
Quote XoceroX (

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Would an SSD make any difference?
Not really, certainly not enough to be worth the extra money. Better to get a high performance HDD (WD Black for example). SSD would only make loading textures from disk faster, but since almost all of SE is procedurally generated, this wouldn't help.