Hi - A friend recently helped me install a new dishwasher by running an electric connection from a nearby outlet. For the extension, he took the power source from the outlet then connected to a metal box which he then ran a connection to the dishwasher. I noticed that the metal box is right next to (in fact) touching the water source to sink. I was a bit concerned about the connection but he reassured me that its OK and metioned that many buildings use the pipes as ground.

My gutt feeling tells me that this type of connection could be dangerous since the cables are exposed if a water leak ever took place. Could you please let me know if this is something I should be worried about? Thanks

This is not something that you want to do. It can create "ground loops" in the house where ground is now not a zero volts. Pipes can vibrate and punch through, being near the box.
I'd correct it.