He lives in Germany as a permanent resident. So he can either appeal, pay the fine, or move back to the US (plane tickets alone would cost more than the fine, let alone the cost and disruption of moving a family).

If he lived in the US, he could likely just ignore the ruling. This would clearly be 1A protected speech in the US, so even if Germany pursued it, they would be politely told to screw off. It would probably be a bad idea to return to Germany though.