I'm not sure I understand the problem correctly. By "dying," does
he mean the flowers are expiring, or the entire plant? If it's the
flowers, I would suspect something like ethylene. Orchids are more
sensitive to ethylene than just about any other plants, except carnations.
Are his carnations suffering similarly?

I am not sure the analysis that was performed on the air would
cover ethylene, as it isn't a toxic problem so much as a flammable gas-
and only a hazard at a much higher concentration (several percent). If
ethylene was screened for, then kick me in the head again, and see what
falls out.

However, if it WERE ethylene, it could come from an industrial
process (unlikely), or agricultural- oranges produce lots of ethylene as
they ripen, but have rather lower sensitivity to the hormone. In fact,
oranges may be treated with a shot of ethylene in order to get them to
ripen to a uniform orange color. Ditto with bananas that are harvested
green, and shipped after being gassed with ethylene for ripening in
transit.

Another possibility is that there is someone with oranges nearby
who has had their plants treated with "Fruit Eliminator" or similar
product that contains ethephon that is used to abort flowers so that
fruits are not formed.

One way to test for ethylene being the problem would be to buy
some flowers, and run an experimental group kept in a chamber/refrigerator
with some potassium permanganate-based ethylene eliminators. These are
safe widgets that chemically react with the highly reactive double bond
between the carbons in ethylene to break it down to gases that don't whack
your flowers.

These things can be tricky to hunt down. Let us know if it works
out for him, would you?

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-AJHicks
Chandler, AZ