So you're trying to get an Article by author GUID, correct? I can assume that AuthorGUID is a field of CMS.Article page type. Try this:
var docs = DocumentHelper.GetDocuments(SiteContext.CurrentSiteName,"/%","",true,"cms.article",
string.Format("AuthorGUID={0}",guid),"",-1,true,new TreeProvider())
This selects documents along with the coupled data (Article in this case), which allows you to filter by coupled data fields.