It's wrong to intentionally cause harm to someone's relationship.
Ken and Bob are talking about Bob's girlfriend one day at lunch, and Ken does not like Bob's girlfriend.
Ken wants to give Bob some advice.
Ken keeps his mouth shut and gives Bob good advice while keeping his personal opinion aside.
Ken tells Bob he hates his girlfriend and tries to sabotage the relationship.
Bob and Ken's friendship is preserved.
Bob gets mad at Ken, and they get into a fight.