The worst case is that you are accused by someone of plagiarism. And that is pretty bad. But you don't ask for solutions, though user Younes provides one in a comment.
Contact the organizers, explaining the situation, apologizing and asking for a correction. The organizers, if they have any experience, will have dealt with submission problems in the past and so expect them to happen.
However, if I read it correctly, the paper itself contains the correct author information in its header. The effect, then, of not having entered the co-author into the system is just some confusion on the part of readers who notice it. Conference advertising and such may include less than complete information for example. So, it may be that the "worst case" is an inconsistency in what appears in conference documentation and in the paper itself.
If the paper is correct there will be less of an impact, except for people searching for papers by your co-author and not finding this one since the conference docs are incorrect. If the organizers understand this, they should have an incentive to make it right.