Mac/Windows Mismatch error- how to fix??

[SuperZtar64]

My friends and I were setting up to play a game of Civ 5 this evening, when one of my friends encountered an error. One of my friends uses a Mac, and apparently for some reason crossplay between mac and windows doesn't work for Civ 5. The one of us that's on Mac has a windows computer at home, but now that he's at college he doesn't have access to it. Our evening plans were ruined.

There has to be some kind of workaround for this. What do I need to do to play with my friend on Mac? We're all Steam users.