The best authority to refute your friend's theory would be an
immunologist/infectious diseases specialist, someone with a sound
understanding of what is and is not known about the differences in the
functioning of the immune systems of people who are, versus people who
are not, sensitive to various allergens, including agents in bee venom.
Your friend sounds like he may have muddled those two cases.  Maybe
Karen's allergist could explain in more detail.  Someone who is
"hypersensitive" could possibly be "sensitized" by one sting, then react
catastrophically to the 2nd one.  Whereas someone who is not
"hypersensitive" might be desensitized by the first sting, so that
subsequent stings produce progressively lesser reactions.

As a fairly new beekeeper myself who does not have an anaphylactic
problem with bee stings, I can only say that during the season my
reactions to stings become less severe, but depend on where I get stung,
the face giving rather severe reactions (which still aren't problematic
for at least 18 hours afterwards and disappear without a trace after
another 1-1/2 to 2 days).  By the end of the summer the rest of the body
reacts more to ant bites and mosquito bites than to bee stings.

Susan Gilbert
Indianapolis