In fact case sensitivity is not meant to permit "xxx" and "XXX" to be at the same time.
It's just one of the many ways to make software lighter and faster (to write and to run), because you don't have to match two characters (or more, in some languages) every time you input one. Of course it's not much more work to do, but it's better than nothing.
Anyway, I think that it's actually easier (and search-engine-friendly) to change the pages link in your customer's website. If you paste here a couple of A tags as example of his links, I can give you a perl regexp command to fix them all in one time.