SQL Server - Query Over Linked Server to IBM DB2 throws an error
Well, I did a lot of searching, found some information on the SQL0204 error, which led me to this support page:

https://www-01.ibm.com/support/docview.wss?uid=nas8N1016377

which has instructions on what commands to run on the server if you see that SQL0204 error.

I forwarded that on to the partners IT guy, who tried it (and a few other commands according to him) and it started working.

I will try to see if I can get him to send me what other commands he ran so I can make this answer more complete.

Edit:
In case the linked page ever disappears, here is a screen shot of the information: