The free viewer does not write to the DWG files, so there is no way to fix DWG files with it. Sorry.
Ask the DWG file original creator (the AutoCAD user) to provide everyone with those missing SHX files so the viewer users can add them to their DTV search folders to avoid this issue. It's what all of us AutoCAD users do when we see missing fonts from the DWG files we get sent by others.