Catelyn had been left her husband's castellan in the North, which means she has the legal authority to order anyone around (which was not unusual for a married woman in Medieval Europe, if the wife was considered competent); beyond which, Robb sent his mother to negotiate on his behalf, and he agreed to the treaty when she presented it to him, so if he had had any objections that would have been the time to raise them. Why would that matter? Frey gave them access to the Twins, and thousands of soldiers, who continued to serve in Robb's army. He committed himself to the Starks' cause. Nothing was conditional on Ned actually being rescued (no lord would ever agree to a bargain like that, since it's totally dependent on things beyond either party's control).