Is it possible to create the interfaces of an app on Windows?

A friend of mine wants to help me with the UI of an app, but he doesn't have a Mac. I know that the most asked question is "can I program without a Mac?", but maybe, just maybe, it is possible to do the UI part without one.

Is it possible to run Interface Builder on an emulator running Mac OS?

Do you know of an alternative for Windows to create XIB files?

Is there another tool to "sketch" the GUI of an app, using the standard controls on the iPhone SDK?

Thanks for your input.