A "mechanic friend" refers to a person who is both your friend and also has knowledge and skills in fixing or repairing machines or vehicles. They can help you with any mechanical problems you have. Full definition
MECHANIC FRIEND SAID DO NOT TIGHTEN WHEELS WITH AIR WRENCH JUST TIGHTEN WITH TORQUE WRENCH... NO BRAKE PROBLEMS SINCE. (carsdesk.com)
Reporting speculation of insiders is saying «My crazy mechanic friend Bill says his friend thinks he saw Sasquatch so, anyway, here's Wonderwall.» (sbnation.com)