Yes, you must carry out the full MOT re-test every time.
You might have watched your mate swap that exhaust or change those pads, but unless he’s also an MOT tester carrying out the MOT re-test procedure, your job isn’t done. You still need to check it all under MOT rules, that means emissions test where needed, and any other checks the defect or related repairs require. The MOT tester is responsible for the MOT re-test, not the person who turned the spanners.