Each Defendant in a criminal matter stands on his/her own, regardless of what the other Defendant(s) decide to do to resolve the charges.  Each Defendant has certain Constitutional rights, among which, is a right to a trial by jury and a right to confront his/her accusers.  Accordingly, if a co-defendant enters a guilty plea (or any other plea), each Defendant has a right to a trial in front of a judge or jury.