You shouldn't share information about someone without their consent.
Melissa sees her friend Andrea out on a date with a guy who is not her boyfriend.
Melissa wants Andrea to be happy.
Melissa has a conversation with Andrea about what she saw and does not tell anyone else.
Melissa tells Andrea's boyfriend about what she saw because she thinks Andrea will be happier if they break up.
Melissa and Andrea remain good friends and Andrea continues to cheat on her boyfriend.
Andrea's boyfriend breaks up with her, and Andrea stops talking to Melissa because she ruined her relationship.