I have a friend that has a bunch of empty jewel cases and wants to know what to do with them. She doesn't think you can recycle them because the plastic is too thick. Any ideas?
I don't think the thickness of the plastic makes any difference. The key would be to find out what plastic they're made of and then find someone to take them. That may be easier said than done.

Donating them to the library as KKM suggests is a good idea. She might also want to ask all her friends and family members if they have any broken jewel cases they would like replacements for. I think most everyone has at least one or two with a crack, broken hinge, etc.
It's kind of interesting that, when CDs first came out, a lot of people said jewel cases were crap (I thought so too), yet in all the years since I haven't seen a good replacement.
The record companies have come up with some things that are worse, but nothing that's really better.