Wow, that reminds me why I abandoned the Macintosh plateform almost 10 years ago. Very limited options in mechanical CAD!
I made a search, there doesn't seem to be a quick, easy and free way to do it.
Believe it or not, if you were working with SolidWorks or even Pro/E, the solution would be quick: there's an eDrawings 2007 Viewer for Mac available for free on SolidWorks web site. If it has the same fonctions as the Windows version, it can view eDrawings files, Solidworks files, Pro/E and DWG/DXF (2D only), 3DXML and STL. Of all these, SE can only export the last one in 3d and it would be close to useless.
An option for you would be to download the eDrawings Publisher for Solid Edge; it allows you to generate eDrawings files (drafts, parts and assembly files) that your customer will be able to open with the viewer. What I don't like is they require registration with phone and zip code before allowing the download.
An easier way for your customer, but it would require you a big purchase ($995): convert your 3d model into a 3d pdf with Acrobat 3D. The latest Adobe Reader reads 3d pdfs and upon opening one, you activate a toolbar with simple tools to manipulate the model.
Yet another option to explore: Google SketchUp Free for Mac. But I'm not sure that SE can export to a format that SketchUp can read. I believe that when I tried it a few months ago, I had to convert my model to ACIS (.SAT), then open in AUtocad and export to 3ds.
Good luck!