My basic question is 'did I break my buddy's brake or did it arrive broken? Here is what happened:

A friend of mine asked if I could replace his disk brake pads. I said I had only done it a few times and it had been a while, but I said I could do it. He arrived having already removed the old pads from the front brake (worn completely down to the metal -- original pads and bike is at least 5 yrs old). I immediately noticed that where the adjuster knob on interior side had a metal tab in the center, the one on the exterior side did not, just an empty rectangular cavity where once protruded the tab. This I thought odd (see picture of red knob with silver tab in middle)

Upon turning the exterior knob to see if the mechanism was functioning correctly despite missing tab, the aluminum disk on the inside of the brake that moves in and out to adjust pad position when the knob is turned popped out of place. After partially disassembling the brake to investigate, it seemed to me that the metal disk that had popped out of place from inside the brake and the metal tab missing from the center of the knob on the outside were in fact once all one piece of aluminum -- i.e red knob engages tab then turning knob moves disk in and out . My theory is that the tab had gotten sheared off from excessive turning over the years, thus resulting in aluminum disk popping out of place. Is my theory about shearing correct? If so, this shearing definitely occurred before arrival in my garage since I turned the knob only a few clicks before things fell apart. It seems my friend needs a new brake. What are thoughts among readers more expert with disk brakes than I as to whether perhaps I somehow am responsible for damaging the brake?