The milk is mucus producing stuff is an urban legend. Unfortunately, many
advisors (in all different fields) are not willing to admit the limits of
their knowledge and when they simply have no more good ideas left. I think
some groups are more prone to this than others but it is true to some degree
of all types. Advisors tend to give good, solid, well supported information
when they have it and gradually move into less well established territory as
they run out of things to say until they start spewing nonsense they read on
the Internet and heard from their neighbor. The "dairy is mucus producing
and should be avoided in asthma" is one such factoid and should be a clue to
get up and walk out of the office. So is the common "drink eight glasses of
water a day" advice.

Some people are allergic to milk. They can have it as a trigger for asthma
as well as eczema. Cow's milk is a common trigger for eczema in infants. It
is often a delayed reaction that does not start until a 8 to 12 hours after
ingestion. A common history is that the person drinks milk with dinner and
then starts wheezing at 3 am leading the docs to diagnose a dust mite
allergy and hound the patient about cleaning the room and covering the
matress. When the symptoms persist the docs assume that the patient is not
being vigorous enough with the dust mite control measures and harps on them.

In this situation, or any where the asthma symptoms seem to have an
undiscovered trigger it is worth trying to eleiminate dairy for at least
several days to see if it helps. Most asthmatics, however, will find that it
doesn't. Advising those people to avoid milk due to some general mucus
producing effect is quackery.