You are exactly right in thinking that the prolapse is typically caused
by diarrhea.  When there is not something firm enough to push against the
intestines over-do their contractions and prolapses are among the results.
With a mild one keeping it clean and lubricated is perfect.  A bit worse
and some gentle Prep H, or some cortizone cream made for that purpose, or
a combo are usually used If it gets bad -- large or dark-red or purple --
then an EMERGENCY appointment will be needed to prevent tissue death.
First a purse-string will be tried.  that is an easy procedure.  Rarely,
more extensive work is needed, but pursestrings usually work well.  BTW,
you are also right in thinking that this can happen and does happen with
diarrhea to ferrets from any source.
[Posted in FML issue 3632]