Are you questioning the action because she was illegal, or because she was a "ringer"?
Sometimes, situations arise where a team is short of players on a particular weekend, so they throw an illegal on the field just so they don't forfeit. But if the unauthorized player is suddenly the best on the team, everybody, justifiably, screams.
Regardless of the reason, yes, it's illegal - but, the grey area is : did the player have an impact on the outcome of the game?