Most of my friends didn't like my ex, my parents didn't like my ex, my grandparents didn't like my ex.
They all told me in roundabout ways or questioned everything he said trying to undermine or prove to me he was bad news, they made it obvious they didn't like him. I then became protective over him, isolated myself, thought they were all against me, limited my time with them and it gave him full control over me as I was very reliant on him.
When the relationship ended they all came out with the I told you so, if only you had listened.
For me I look back and think if they hadn't have made everything so awkward I might have noticed things were bad earlier or felt I could tell someone how I felt and what he was doing but instead I was stubbornly trying to prove they were wrong and on the defensive about him everytime I saw them.
I personally wouldn't come outright to someone and say your partner is bad news, I think I'd do the "did you hear or see the story about" act and then gently let them know I'm there to support them no matter what.