You cannot save a .RVT file directly, you can however save as an .IFC file which can be imported into Revit (or any other BIM package).
IFC is 'the' BIM format it is platform and software version agnostic.
If your client wants an ACAD file ArchiCAD does an excellent job of saving dwg files that are very usable in AutoCAD. You do not need to have Autodesk software installed to save to this format.
If you have downloaded the improved Revit interoperability plugin, this is a plugin for Revit not ArchiCAD and helps importing of ArchiCAD elements from IFC files and mapping them to the correct Revit entities.
Scott
