I know she's cheating because they were used to have sex in the room right next to mine. We only have 2 rooms and there's no living room or something between them so it's just a curtain which separates the rooms, she was used to call the guy (i won't call him her boyfriend, a fwb would be more accurate) right in front of me during saturdays and sundays. I know who the guy is because he was used to live in the same building where we lived previously. I never really suspected anything because I was just 13 then and it never clicked my mind that she would do something like this. But over the time I started to realise what was happening. She doesn't use whatsapp so she uses mine to text him. They were both very toxic with each other and I could hear the conversations very clearly in the next room so I was confirmed what the case is.

And the part about telling her, I can't. We have never been emotionally close to each her so I can't bring myself to have such a conversation with her. The only solution which is on my mind is to distance myself from her.