Hi half-life Sounds like a frustrating situation.
She has great difficulty to act even mildly assertive. Even when she was harassed and distressed, she is not capable to object but rather turn inward and suffer in private.
Is she getting any counseling? Looks like you may have your work cut out for you to make sure your children don't learn this behavior.
I drafted her a text message to end this unambiguously.
If she sends it, she needs to make sure she takes the necessary steps to send the text anonymously, so her new phone number won't be discovered. There are ways to do that, but she would need to take certain steps to accomplish that.
It distressed her so much that she changed her number and wanted to avoid him.
Did she try blocking the sender on her phone?