My neighbor is cheating on his girlfriend. I feel obligated to tell on him. How best do I go about this?

My neighbor is cheating on his girlfriend. I feel obligated to tell on him. How best do I go about this?
He is a player to the core. I mean like, I see him come home with different girls, even whores.

In my neighborhood, everyone usually minds their business and I've been minding my business but I'm tired of doing so because his girlfriend is around for the weekend. That's the only time I don't see strange women around.

She is usually quiet but I was on the porch today and somehow we struck a conversation for the first time and I found out that she is from my hometown. I now feel obligated to tell her the truth. Especially when she started gushing about how she loves him and hopes to end up with him.

She looks like she needs someone to talk to and I was somewhat tongue tied but I feel the need to tell her tomorrow. Especially as he is not around. She revealed that he was away on business (which I think is a lie) and she felt like just relaxing at his place so he gave her the keys to the apartment. She even revealed that she was getting bored and wants to leave tomorrow to her home but that he told her to wait for him. She said if she leaves, he will be very angry, so she won't. She's so alone and I feel sorry for her.
How best do you think I should break it to her?
My neighbor is cheating on his girlfriend. I feel obligated to tell on him. How best do I go about this?
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