Have either of you you tried hosting a game?
Did you try joining a new game during character creation?
Please email supportdos@larian.com, with a description of the problem and the report.zip file generated by the
D:OS support tool (for the PC version). The report will contain system and game information, as well as your saved games. Since the saves are not relevant, you can greatly reduce the file size by deleting the save folders from the zip.
If you and/or the host can check their '..\Divinity - Original Sin\Shipping' folder for any CrashDump files (add them to the report.zip, or zip them separately) they may help tracking down the problem.
For Mac, crash reports are collected in the 'Console' application. To get system details in Mavericks you can use 'system report' from the 'about this mac' apple menu item or the 'system information' app (utilities folder); in earlier versions use 'System Profiler'.