Definitely tell her, but as others have said, be prepared for it to be a slow burner.
Depending on her personality and level of feelings for him, she may respond dismissively or even angrily, may ignore you etc.
But it will plant a seed. It will inevitably be something she thinks about, so will likely cause her to be more vigilant, even if that is on a subconscious level.
Best case scenario, it will also prompt her to do a Clare's Law.
Another thought, have you heard of the Are We Dating the Same Guy Facebook sites? If you feel confident to do so (please don't put yourself at risk!) you could make a post about your ex, to warn any future dates to do Clare's Law. The sites do work - I found a guy my friend was dating on there, and even tho she actually decided to continue dating him (he wasn't an abuser, just a serial shagger) it did prompt her to set clearer boundaries with him and proceed with caution....
Good luck OP, and well done for getting out of this relationship, and for having the courage and goodness to do what you can to protect other women - you are in no way obliged to, but it's lovely that you choose to!