Re: "nd what's the problem with an ally (*) buying a potential adversary's kit?"
Most certainly not. You can be sure that the export versions won't be as capable as the Russian home forces versions (not to mention having some hidden capability to render the export version unusable or materially ineffective if so required).
So we get second rate Americal weapons like the F-35, while theire top aircraft do not get exported. Maybe the EU should put sanctions on countries that didn't buy the Eurofighter.