Should I tell this girl that her boyfriend is cheating on her?

this morning my friend told me she hooked up with Tim, a guy from my English class, at a party last night. Tim has a girlfriend, a very nice and sweet and pretty girlfriend. I feel bad that she is getting cheated on and want to tell her. if I was getting cheated on and someone else knew I would want them to tell me. I want to say something so then she can make the decision of whether or not she wants to stay with Tim. it could break up their relationship but that might be a good thing since Tim is a cheater and a liar. should I tell her?
Should I tell this girl that her boyfriend is cheating on her?
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