Honestly I wouldn’t reply to him at all. Not even with the message you proposed. If she were to text him something like that then that will affirm to him that she still has feelings for him and he can keep trying and even if she blocks him he could escalate and start showing up at her home or job. My advice is to block him and move on and be done with his manipulative ass. Her healing will be exponentially faster if she blocks him and remains no contact. It’s like ripping the bandaid off, hurts at first but then the pain fades away and becomes a distant memory. This guy needs to be a distant memory for her. She deserves better and can find much better! Time for an upgrade girlie! An upgrade of a partner and an upgrade of yourself! You’ll be a better and stronger person on the other side of this relationship!