(Goes without saying, but IANAL) It's my understanding that in the US you can be sued by anyone for anything. That doesn't mean the case has merit, but it does mean you're obligated to defend yourself or lose by default. This is often the mechanism by which patent/copyright trolls put pressure on their 'mark.'

It sounds like the plugin author isn't located in the US, so this may be moot, but countries colonized or invaded by European countries or the US often have similar legal systems.

I doubt they'd win in a fair fight, but I also doubt the plugin author can afford to put up a fair fight.