he has a girlfriend

ok so i started talking to this guy in may. nothing serious happened from may-june, jus exchanged numbers/snapchat. sometime this month we hooked up two times. after the second i found out he has a girlfriend. at first i told him that im not gonna be the girl he cheats on his girlfriend with and i tried to cut things off. after like a few days he hmu again and idk i gave in (i dont need a lecture on why it's completely wrong believe me i feel so guilty). we've almost hooked up a third time since i found out he's in a relationship.... but things never seemed to workout (probably for a reason or its a sign right). anyways ever since i found out he had a gf ive felt super guilty but let lust get in the way. ive finally found her social media tho and im contemplating messaging her and telling her he's cheating. i know if i was in her position i'd want to know asap so i could leave him... idk whats stopping me. my heart hurts for me and for her mostly.. idk what to do? is it my business to tell her??? because when i asked him why he's cheating he told me its "none of my concern"... i just think i deserve better than being a side chick especially after everything i've gone through with guys and i definitely think she deserves better. please help... should i tell her? im literally having an anxiety attack over this im so stressed i feel like she should know but i know it'll crush her