Are they at least on a current support plan??? Sounds like there are several topics of conversation in your future.....
Dump everything into Excel. Add the CI item table to a sheet. Add the alias item table to another sheet. Write an if statement formula using a vlookup to look in the ci item table first for a value. If N/A, then use a vlookup to look in the alias table for the item code. Something like below.
=IFERROR(VLOOKUP(C1651,ci_item,1,FALSE),VLOOKUP(C1651,alias,2,FALSE))
This is what I use at a client for their physical inventory count. They scan the bar code tag on the tire which is the manufacturers number. Their ci item number is different.