I don't think anything would happen from it - there is a saying in danish which translates to "its easier to beg for forgiveness than get permission" (it sounds better in danish). However, to be sure that it is only for personal use you might get the client to sign a contract saying so (or at least confirm via email) ... or maybe you could actually print a poster for him with the map - that way he won't get the files.
As long as its not for mass reproduction I think your safe. Most companies ignore fan-art ... Star Trek hunted fan pages on the internet some years ago throwing lawyer letters about, but it went away. It just showed their lack of understanding the web.