"For OpenMobile use only" is a bit against the "open" spirit isnt it?
As long as he does not actually release the dll, he is well within his distributing rights given by GPL to release a plugin using that dll, since OpenMobile GPL'ed the interface to the dll.
And fortunately, you have provided that DLL download free to anyone, so all he has to do is tell people where to get it, which again he is well within his rights, and since he isn't distributing it, it's on the end-user whether or not they agree to your EULA.
Edit: A caveat, this is for more lawyerish people than me to argue, but I'm fairly certain it is not against the rules to use a GPL plugin with a closed source system, as long as the interface is freely available which I think CF's is. If not, it would behoove the CF developers to make their interface public domain so as to allow GPL plugins to be developed.
Stretching it even further, if he were to write a white paper on how OpenMobile interfaces with the pandora DLL, someone could come along and take that and write their own plugin based off that (See: Clean room reverse engineering), and then have either a closed or open source pandora plugin which uses your DLL. Again, not providing the dll itself. You're not going to be able to keep this one to yourself for long, you knew that.