If the break is in the humerus (the upper arm), then your vet is correct
in that a cast will not be effective.  The cardinal rule of casting or
splinting is that the joints above and below the break must be immobilized.
If not, the cast actually acts as a weight on a pendulum, worsening the
motion of the break and pain at the site, and resulting in a poorer
knitting of the bones than if one was not applied at all.  While the
joint at the bottom (the elbow joint) can be immobilized - the only way
to immobilize the shoulder joint would be to put the ferret in a cast
around the trunk, or a small body cast (a humorous picture, but not a
workable solution in ferrets).
 
Casts and splints that work well are applied to breaks of the lower
extremities, which are probably even more common.  Breaks of the humerus
and femur are usually best treated with internal fixation (plates, pins,
and wires).  If that is not an option, then I believe a better option is
truly simple cage rest.  You may be surprised how well a young ferret
will heal.  I wouldn't guarantee full function, but I've been oft surprised
by ferrets with orthopedic injuries.
 
With kindest regards,
 
Bruce H. Williams, DVM, DACVP
Join the Ferret Health List at
http://groups.yahoo.com/group/Ferret-Health-list
[Posted in FML issue 3434]