Cathy's friends see her as an emotional "downer." when she arrives at a party where everyone is laughing, the laughing and fun end. this change in her friends' behavior illustrates

The change in behavior illustrates a mood linkage.

Mood linkage refers to a psychological phenomenon when individuals absorb the emotion of other people around them.

In this particular case, cathy's friend feel the emotion that displayed by cathy and unconsciously absorb it which make them feel similar emotion without them realizing it.