Honestly, the best thing to do would be to build another rudder at some point. This might be daunting if your friend did not build the plane, but any RV builder could help him and have it done in no time.
Early RV rudder and elevators were made with .016 skins. They can be prone to cracking unless built correctly using a few tricks. Here is a summary...
http://www.romeolima.com/RV3works/Ai....htm#Empennage