My girlfriend has a 2000 jaguar XJ. She had one burned out low beam, and when the mechanic replaced this one bulb, both low beams barely threw any light. She drove a few days with the high beams on to compensate, but now they aren't working. Could the mechanic have screwed things up? Installed the light wrong? etc. He claims she now needs a new part which will be hundreds of dollars...all for a burned out headlight. The rest of the lights were fine until he replaced the one bulb. The other low beam was plenty bright. In fact, it was brighter by itself than the two low beams combined after he replaced the one bulb. I wondered if it could have been a ground issue or something to that effect.
If all the mechanic did was replace the headlight that went out for some reason and the lights did not work correctly afterwards, then i would say the part was probably bad to begin with or the replacing the bulb would have fixed it the first time.
It is possible, You should have had him remove the bulb after noticing that it did not fix it right but you probably would ave found it needed the same part as what the mechanic is telling you that you need now.