A paper with copied material
First, deal with your collaborator, then notify the conference asap. If they figure it out and you then are like "oh, yes, sorry, I know, I will change it." you are in a bad place. Not only did you waste the reviewers time with a version you knew was wrong/has to be corrected, your late excuse might also not seem honest.

If, however, you inform them yourself, apologize for the inconvenience and the mistake and give the corrected version, they might well forgive you.