I used to order a lot of custom things from China. Took a lot of trial and error to find good suppliers. They only accept bank wire as payment, so they risk little other than repeat business if they choose to be sloppy.
I think from memory the inside of the circle (which was cast) was not perfectly round. From memory the original Porsche part was titanium, and this replacement was aluminum. But I might have that backwards.
What doesn't sit right with me is the fact that for a small run, the price of even an aluminum casting would absolutely motivate me to get it right the first time. At the level of volume that the guy would have to be operating at, I would be test-fitting every part to a car before sending it out the door.
I feel like the relative scarcity of air-cooled 911s coupled with the difficulty of machining the part correctly as a third party is why the part itself is so hard to come by. If a person were motivated, though, there is definitely another way to make a suitable replacement more cheaply and easily with a modified design. That's provided someone is okay with not having a completely factory 911.
Edit: I actually looked into it a little more- it looks like the part is probably machined from casting given that there appear to be four stators which also locate the center mounting point. That's where the concentricity would be set; not by the casting, but by the machining of the center flange. So assuming that the fan's mounting points are within spec, I would say that your friend's part could just be chalked up to machining process that leaves something to be desired. This is actually a trickier part if machined from casting (without the appropriate tooling), but actually pretty simple (and wildly expensive) if machined from billet.