There’s difference between courtesy and friendliness. China is showing the former. It’s a favor that costs them nothing.
Turkey is an US ally, they cannot be trusted with cutting edge technology. Selling anything to turkey is as good as selling it to Washington directly. If a J-31 that isn’t completely gutted like the Iraqi t-72s is sold to turkey, I give it 5 years tops before US rolls out an improved joint strike fighter.
or in reverse - and that's exactly the point for the F-35 issue - since in the current situation no-one can guarantee that F-35 technology would end up in Russia, Chin or whatever.
But stop this off topic ... back to the FC-31.