So long as the problem is the defective ABS ECU the light will not come back on. Think about it for a minute, why is the light on? Because the control unit is turning it on because it has a fault stored in it. If you replace the control unit with a new one the new one should be cleared of all codes before it goes in. Now if there is a problem elsewhere (not likely) then it will see it just the same as the last ECU and turn on the light. This is where diagnosis comes in, if it is being repaired by someone who is competent and knows what they are doing they have already diagnosed it and know what the problem is. If not then they are talking a shot in the dark and hoping to fix it. Chances are that it is a bad ECU but without pulling the codes and diagnosing it properly you will not know until you change it out and take it for a test drive.

Good luck, Mark