The mechanic should be checking the amperage output of the alternator. It sounds like there is a good chance that the problem might have something to do with replacing the starter so grounds and connections around the starter and alternator, including the jump terminal(if the diesel has this) connection.
Looks like the diagram Wayne posted
shows the light old school to the alternator so that explains why it can be dim.
I don't think it's the case here but the wrong bulb for the indicator can cause this kind of thing.
Maybe the wires got mixed up ??