First make sure that the AAF from the converted XML has the same source information that was used in the transcode – it will either be in the Source File or Tape columns. And they will hopefully match the original file names although there is a very good chance that the original clips will be tracked via the “Source File Column” while your AAF has it in Tape – but you need to confirm that.
Either way – highlight the clips you want to relink it to in all the bin(s), and then highlight the seqeunce. Right click the sequence and do a “relink”. In the relink dialog window, check “relink to selected” and in the “relink by” section, choose timecode (default) and “Tape name or Source File” from the source name menu. I would deselect “match case when comparing source names” and then click OK.
That “should” do it barring any other renaming issue that may have occured on its trip to FCP and back.
This assumes that your client was using your media… if they just linked to the files directly, then FCP does not use the entire filename, but the truncated 8 character version that is listed as REEL inside the QT header… that is another problem considering Media Composer does not extract that Reel ID via AMA and unless you imported the ALE that was created in camera when shooting, the process will be quite manual…
Michael