The controller did something so incredibly bizarre and outrageous that I finally saw what they were doing, and I could never un-see it. Others had spoken truth to me a few times over the years, but recognised that they couldn't change my mind for me, so they didn't press the issue. The controller was trying to isolate me from all those others anyway, so if they had pressed the issue the danger would have been that my controller might have then been able to say "See, they hate me and they're trying to break us up" and got me to further cut contact with them. (In fact, this had already happened a few times.) As it was, when I realised what was happening and asked these others for help, they had been waiting ready for me to ask, and sprung into action immediately. One drove all the way across the country to help me out.
So yes, if you know someone who is a victim in a controlling relationship, then gently speak truth to them, don't push too hard, but be ready to give them meaningful help as soon as they come to their senses and ask for it.