Should she give back the ring?

My sister dated this guy for almost ten years. They were engaged for two. She caught him cheating and forgave him. They tried to make it work but my sister had problems trusting him again so she literally stalked him and read all his personal stuff. After some time she found out he was cheating again with a different girl. This time she went crazy and told both of them off and made quite the scene outside his apartment.

Do you guys think it is mentally healthy for her to keep the ring? Or should she just give it back and end this for good?