Is butting out of a bad relationship enabling?

I’ve known young ladies/guys who have gotten together with guys/women who were clearly using them.
You risk losing them in your life if you say something.
You also risk losing them if you say nothing and they stay in this bad relationship. Over time the bad person will notice you see who they really are and will persuade your friend/relative to avoid you, or your friend/relative out of shame or anger will avoid you.
I’ve had relatives introduce people who you could just see clearly in their behavior that their new boyfriend/girlfriend is someone you wouldn’t trust for a minute. Then they ask you that question.
What do you think? You know they don’t want to really hear the truth. If you tell the truth, sometimes they stay in the relationship because they want to prove you wrong.
However, if you tell a lie. Aren’t you enabling this person, because if they had any doubts, you may have just erased the doubt.
If you had a friend with an alcohol problem and he asked you if you thought he was an alcoholic, you wouldn’t tell him no. Your just a social drinker or buy him the next round.
So why do we sometimes avoid being honest with the people we care about?