It happened to my daughter.
She is always helping people around her but in the end, got stabbed in her back. There was someone in school that she always giving a helping hand whenever she needed but unfortunately for my daughter, she was stabbed in her back without knowing about it. Later someone told her about it then she realized. Even though that person did that, she forgave her. But that did not stopped that girl from doing it again and again.
As a parent, it is disheartening but we told her that it is okay. One day she will receive the effect of Karma & Merits for what she had done.