Just sitting here at my desk, wondering where that lunch I just ate is
going. I was told by a friend who is well-built that while he is at
work in his office, he often flexes his ab muscles, arm muscles, etc.
He credits this for helping him tone his body. Is there any truth to
this? Does flexing while in a stationary, non-exercise environment
help tone muscles?

stanfordbradley...

In short, yes. There is a form of exercise called isometric
exercise, in which a muscle is tensed by exerting it against
an immoveable object, or against another muscle. For example,
you can hold your arm with the elbow at a right angle, and 
tighten your muscles until your arm shakes with the exertion.
You are actually opposing your bicep muscle with your tricep.
The shaking is actually indicative of poor tone, and regular
use of this form of exercise will effectively improve the 
muscle tone of both the bicep and the tricep, resulting in
less shaking with the exertion.
 
This post from the SFC (Scientific Fighting Congress) forum,
on W. Hock Hochheim's martial arts website, confirms this, 
and examines the topic in some detail:

"...when properly used, isometric exercise can offer excellent
 benifits in a short period of time. You will see increased
 muscle tone and hardness, which offers some impact resistance
 to punches and kicks. You may see some immediate minor
 strength gains which carry over into combat training.
 However, it needs to be emphasized that isometrics are
 not a replacement for other forms of exercise, they should
 be thought of as additions to your current prorgam."
More on the page:
http://www.hockscqc.com/discus/messages/7/153.html?1072230657

He notes that research done since isometrics first gained
popularity in the 60s has confirmed the original claims
that led to its notoriety, i.e., that:

"...one 5 second isometric muscle contraction per day could
equal strength gains of several reps of a weight training
movment".

He notes that the research has also found that the ideal 
length of contraction should be 6-10 seconds, vs 5 seconds.

So your friend is on to something quite beneficial, even if
he doesn't know the name of it...  : )

Please do not rate this answer until you are satisfied that  
the answer cannot be improved upon by way of a dialog  
established through the "Request for Clarification" process. 

sublime1-ga

Additional information can be gleaned from an exploration of
the links resulting from the Google searches, outlined below.


Searches done, via Google:

"muscle tone" "isometric exercise"
://www.google.com/search?q=%22muscle+tone%22+%22isometric+exercise%22

good answer, pointed me in the right direction