There was never an incident where the set actually fell down. They may shake a lot, but if a set did fall, it would have definately been retaped. It's always been very distressing for me to find that when most people remember DS, they always remember the bad points of the show, and never the good. They seem to always have some ridiculous memory of a blooper in their mind, like Jonathan Frid's teeth falling out, or some other such thing that never really happened. Never the good things are remembered!
In any event, there was in episode # 1230, (1841 PT) a seance scene in the drawing room where the main cast is seated at the table. Alll of a sudden, a stagehand reaches out to close the drawing room doors. He does it rather quickly, and very loudly too. So much so, that it sure sounds like a wall collapsing. Perhaps this is the scene your friend remembers?