Restore .mdf data file for a File Group in SQL Server
The only way to restore a single file group is to run in all the logs so it's up to date with the rest of the files.  This would of course run in the bad data modification you're trying to prevent but it's necessary to ensure consistency throughout the database.  You'll need to restore it to another location and move the data over.

http://msdn.microsoft.com/en-us/library/ms178099%28v=SQL.105%29.aspx