As long as the date the parent signed and the date the witness signed match, I don't expect anyone to have a problem. I had a situation a while back where the release got filled out incorrectly. I emailed a copy of the release to the model's father, who filled it out correctly and mailed a physical copy back. I've also had a couple of cases where a witness wasn't available, so the model took the release and signed it later in the presence of a friend. Also not a problem as long as the dates match, even if they don't match the shoot date or the date when I signed as photographer.