What does it mean when someone is called a ringer?

This means that this person is just right, or fits the bill, is the best for the job.

Perhaps but in sports a ringer is someone playing for a regular legal player but is not in fact a member of the team. Usually a superior player or a professional playing in amateur leagues, they are illegal and teams using them must forfeit the game if caught.

A ringer is also someone who is a double for another person, an exact duplicate, coming originally from a horse substituted for another of similar appearance in order to defraud the bookies (called a ringer).