In the Seinfeld episode "The Race", Duncan Meyer threatens to fire Jerry's girlfriend unless Jerry agrees to repeat a race Duncan lost to Jerry in high school. It seems to me that firing her over something like that would have Wrongful Dismissal written all over it, or is there something I'm missing?
Yes, he could be sued, but anyone can sue anyone for just about any reason - the question is would he win in court? Most employment in the US is done on an at-will basis, meaning either party can terminate the agreement at any time for any reason, so long as that reason doesn't fall under a few, very specific categories. Wikipedia lists a few protected reasons on the page for Wrongful Dismissal:
Reporting a Violation of Law to Government Authorities
Employee's refusal to commit an illegal act
Employer is not following the company's own termination procedure
This isn't comprehensive, but Duncan's threat to fire Jerry's girlfriend because of a feud with Jerry doesn't seem to fall under any of these reasons. She could absolutely sue if fired, but it's not clear that Duncan would have actually violated any relevant employment law, so he might well win the case.