What is a good FREE graphics viewer for Iges & Step files?

Why don't you give them eDrawings and then they can view/measure SOLIDWORKS files directly. No need to make any Iges or Step files. And they won't be able to change the SOLIDWORKS files.

Geomagic Verify Viewer 64 opens almost all neutral files.

Grabcad.com (Workbench area) works for me.  It can read native Catia etc... and then it will download it as x_t, stp, etc...   I use it as a translator att imes, but it can also be used to view and measure nearly any format cad file.