Hi Jim,
Are you using [ME] filtering in SharePoint already to show users only documents that are created by them or assigned to them?
A simple thing you could do would be to add a field either to the document (in InfoPath) a simple checkbox where checked = True = Favorite. This would allow each document to have a favorite true/false but it wouldn't allow the document to be favorites for any number of people because it's a single field. You could also add a column in SharePoint only with the same purpose, but again it would only allow the form to be marked as favorite which would be a global favorite for the form, not just for that user.
However, if these documents are always created by the person who would need to mark it as a favorite this might work - you could set the condition in the filter in SharePoint to be "Created by [Me] and Favorite = True" or something like that
Hope that helps a bit
Ted