How had Helen "gone mad"?

Helen was under trememdous stress and suffered a hallucination. Unfortunately, her hallucination made her mistake Randy for Mark. Lib explained this to Randy as a temporary fantasy.

"Helen's not psychotic. She's under terrible strain. She let herself go, but only for a moment. She indulged a temporary fantasy. Now it is over. Now she will be ashamed of herself. The best thing you can do is pretend it didn't happen."