First you need to determine where the leak is. Open the box for this zone is it always full of water? If so turn off the water and let it drain. Turn the water on when empty and determine where the leak is. If always full of water you Have to make sure the solenoid is screwed in snug. If so, It is possible you have dirty or bad diaphragm. You can take the screws out of the face, probably 4, and inspect and clean the diaphragm. It only takes a speck of dirt to stop it from closing fully. Put it back together and test it. If still allowing water through you may need a rebuild kit which are quite inexpensive. Depending on valve probably around $10-$20. Once you have the valve fixed fix the leak.

Regarding the wiring and why it comes on with other zones I am not going to even guess. You will need your friend to show you what he did and/or correct it.

Good luck.

SCG