My best friend cheated on his girlfriend with my sister?

My sister told me about it because she didn’t think he had a girlfriend. I feel bad for her because his girlfriends super sweet and good to him. I think she’s been loyal but she does seem to have some issues going on.

When I confronted my friend about he be basically told me that it was because bla bla bla. He just got bored and carried away and not to tell anyone. I told him that it was a horrible thing to do and that his girlfriends sweet. To at least break up with her first.

Should I tell his girlfriend? Or am I being a bad friend to tell anyone like he asked me not to.

My best friend cheated on his girlfriend with my sister?
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