If there is no clear coat on those cases/covers, to get the best results he should consider removing them and buffing them out on a bench style buffing wheel using emory compound and polish sticks for cutting, polishing and finishing aluminum. As with polishing anything else, he may find that the least aggressive polishing or finishing stick will do the job just fine. On the other hand, if the salt damage is deep enough it may require some sanding first. Grit would depend on the severity of the salt etching of course.
I'd recommend not sanding with anything more coarse than 1000 grit and continue sanding down through 2000 grit then finish with 3000 grit. After sanding the cases, use an "emory compound stick" with the wheel on the bench style buffer as a first step, then follow that with an aluminum polishing stick working slow and overlapping sections against the buffing wheel so that the polish on the wheel breaks down leaving a nicely shined look.
You'll find that using a rotary buffer like what is used for buffing paint will not be fast enough to produce any really nice results. This is really time consuming, tedious and dirty work.
If the cases are coated and the coating is compromised. Aircraft paint stripper will remove the coating with a ton of elbow grease and then go through the sanding and buffing steps as outlined above.
In case you don't know what type of buffer I'm talking about, here's an image of the style of buffer that will yield the best results for motor case/cover polishing.
And the various compounding/polishing sticks.