They would need guaranteed non-extradition and possibly a lot of protection so they don't get abducted by the US, but that sort of legislation would never sail under the current German government.
Treason is a crime in Germany, too, and whether the result is politically opportune for Germany couldn't officially play a role in front of a Oberlandesgericht court when ruling on an extradition request by the US. Disclosure of spy activities certainly would be a crime if committed in Germany against Germany, so extradition for such a whistleblower would have to be stopped at the political level, which is possible, but that's not something I would want to depend on. The US could exert a lot of very painful pressure on a German government, and as the Snowden affair shows, broad public support is far from given. Might be easier to weather the scandal from giving in than the consequences of sticking to principles.
