I do not know why age difference matters, since based on your description he is not a minor so there is no statutory rape. Outside of statutory rape, why is her being ten years oolder even relevant?

The issue he is having has a clear and simple solution and I am puzzled why two adults, you and your friend, have been unable to come up with it on your own. Just have calls from her number go directly to VM. Eventually she will get too frustrated leaving msgs and not being called back.