"http://" prefix is automatically added by the Insert Hyperlink Dialog when the provided URI is to local path or mapped drive, e.g. "Z:\temp".

Workaround 1: Set HyperlinkPattern to something that matches file paths, e.g.:
(this.radRichTextEditor1.RichTextBoxElement.InsertHyperlinkDialog as InsertHyperlinkDialog).HyperlinkPattern = ".*";

Workaround 2: Insert such paths with the "file://" prefix.