Do you tell her he’s cheating?

Okay so I’ve been seeing this guy we’re just friends that have sex but he claims he doesn’t have a girlfriend…but when you go to his social media accounts it says there in a relationship and have been for over 2 years and they are active together on social media I like him so I don’t want to tell her we have been having sex and he’s cheating on her because then I am afraid he will
Never talk to me again or should I just let it go and do the right thing and tell her or what if he really doesn't have a girlfriend? Please help what should I do!
Do you tell her he’s cheating?
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