First I'm no lawyer.
The person who is sued should contact the
EFF.
In general it is probably to their advantage that the suit is based in America, because the American law system has a very strong First Amendment that gives you the right for free speech where some European law system emphazise more than the American system on protecting people from wrong accusations.
I also don't know what would happen when the trial would succeed, maybe not entering the US anymore would be enough.