My ex-girlfriend texted her sister in the name of her best friend, asking her to persuade her to go back to school. As a result, she was very angry and asked me to give her an explanation.

In fact, I think this kind of thing is none of your business and should not be persuaded by you. Is unreasonable. It's better to say it directly in your name instead of borrowing your identity. Although we broke up, you regard her as your sister, hoping that she can go to school well and achieve something. I feel much better.