I am looking for a short story that my friend read many years ago,
perhaps 20-40 years ago.

It was a story about a faceless clerk in Manhattan that dreamed about
riding on the 2 statues of lions outside of the 42nd street library in
NYC.  The clerk was a nobody in his job, and people frequently ignored
him, sold him products without making eye contact, etc.  The clerk
began to wonder if he truly was transparent, and realized that if
nobody could see him, then he could ride on the carved lion outside of
the library.  He got on the lion and a crowd of people began cheering
for him.  It is a poignant reminder of human feelings and emotions.

Your story is "The Vanishing American," written by the great fantasist
Charles Beaumont.

"The Vanishing American... the story of a man who never asserted
himself to anyone until one day he turned invisible. He regained
visibility when he fianally decided to do something he always wanted
to do but was always afraid to: 'Ride' the Giant Stone Lion in front
of the New York City Public Library."

GardenWeb: What's your favorite Twilight Zone episode?
http://glyphs.gardenweb.com/forums/load/circle/msg011500408410.html?69

A small portion of the story may be read here (scroll down to the
section with the heading WHY CAN'T OTHER PEOPLE SEE MR. MINCHELL?):

James Madison University: Inquiry Activity 5-1
http://web.jmu.edu/gedproject/printing/volume_three/Vol3_LP5_IA_5-1.pdf

If you'd like to read the story in its entirety, it appears in "The
Best From Fantasy and Science Fiction (Fifth Series)," which is
readily available (used) at a reasonable price:

Amazon: The Best From Fantasy and Science Fiction (Fifth Series)
http://www.amazon.com/gp/product/B000B5SJXE

My Google search strategy:

Google Web Search: "the vanishing american" charles beaumont
://www.google.com/search?hl=en&q=%22the+vanishing+american%22+charles+beaumont

I hope this is the correct story! If it is not, or if anything is
unclear or incomplete, please request clarification; I'll be glad to
offer further assistance before you rate my answer.

Best regards,
pinkfreud