Marcel:But the question is, do you get them to listen to reason before or
after you have let Fred interact with their dog?

Depends on what you mean by interact. Usually it's before Fred comes
close enough to be a problem for them, emotionally. Sometimes the fact
that the other dog isn't showing the usual signs of aggression that the
owner is used to kind of throws them off and they start to realize on
their own that there isn't going to be a problem.