David Musgrave takes a look at making one to many table relationships behave like one to one relationships in Report Writer. Essentially he is flattening the data for reporting. This common technique for data warehouse and business intelligence use and Dave shows how to apply it to Report Writer.
Flattening Related Data for Report Writer
Archived post from 2009. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.