Heterogeneous extranet woes

My brother-in-law is having an issue with his office network. He has a mac laptop with OS X 10.4, I believe, and he wants to be able file share with his secretary’s WinXP machine (again, I am guessing that it’s running WinXP). From within his office LAN, he is able to do this, but he is not able to do so from home. A guy he hired was unable to sort out the problem after several hours of trying. I have a CS degree, but that doesn’t mean I know much about networking. Any thoughts?

Thanks for your help,

This looks like it might be a very similar scenario as is already being discussed here: