What is your relationship to your partner? Wife? Girlfriend? Mother?
If it is neither wife nor girlfriend... then what's it to you? She should be allowed to flirt if she likes. You said it started out as a friendly match. By the way, was the argument by any chance between you and the dark swarthy manly type opponent?
If she is your wife or girlfriend, and not knowing what type of relationship/rules you've established... nailing the guy with an overhead IS legal.