As I've been interviewed recently you'd think I'd be able to help, but I got asked a bunch of really annoying "what does X keyword mean" questions.
I suggest asking them about C# strings, (like why "this " + someint.ToString() + " is stupid" is a bad idea)
Also you could ask them how they'd marshall a pointer by p/invoke. That'd be tricky methinks.
But definitely ask them about garbage collection, and if they know about generations and what is the best amount of time to retain objects etc...)
But mainly I'd ask them about OO principles, design patterns and stuff like that. Picking up C# is a week or so's work for a programmer who understands the principles.