Should I Tell her ?

My best friend and I have known eachother for around 5 years, recently we've been hooking up at my house. He comes over quite often. He does have a girlfriend though. I sometimes feel bad about it, but then I get over it. I am madly in love with him 😭 I get to the point of almost telling his girlfriend that he's cheating on her. Then again , i think he should be the one to tell her. But i dont know what to do. Please Help 😭 ***UPDATE*** We've cut off all sexual "relationship" we had going on and have kept it as it was before , as best friends .