If someone admits they were at fault in writing at the scene of an accident can they later retract it?

they can retract anything however , it would be tough to make a retraction stick in court or with the insurance company. I hope you have the written/signed retraction from the scene and made note of it in a police report and when you reported it to your ins co. . If you do , ask them to give a written retraction to the police or court then have them arrested for making a false statement, and push for prosecution