RE: Futaba 6xa troubleshooting
As long as your friend's receiver is either Futaba or Hitec brand, then just make sure the crystals match. Your Futaba transmitter will not communicate with Airtronics or JR brand receivers. Now, assuming that your friend's receiver is Futaba but on a different frequency (different crystal number) you can pull the crystal from your receiver and plug it in to his receiver and test. It is legal to change crystals in the receivers but not in the transmitters.
There are a myriad of other possibilities, but let's start simply and work toward a solution. I am betting on a bad receiver battery. That is the most likely suspect and the easiest to isolate and fix.