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It's kind of a reach to blame a brake job for a check engine light but not impossible if he had the hood open. You going to have to get the computer scanned to retreive the codes stored when the light came on. That will point you in the right direction. If the car is 1996 or newer, then AutoZone can scan it for free for you. If it's older. post the year, make, model and engine size and i'll see if there is a manual way to retreive them on your vehicle.
You might want to just take a look over it and see if you see anything unplugged first.