Should I tell her he cheated?

So I'm not sure if I should let his girl know or not. I don't know her but I know her boyfriend all to well.As a matter of fact he cheated on her with me. Me and him have history with each other and so last weekend he came to my house and well cheated. My history with him runs about 2+ years and I don't know if I still care for him honestly. But here he was before his girlfriend and him made it official giving me the whole I'm loyal crap.LIES I brought that up to him right before we started but he insisted it didn't matter because he was breaking up with her...it's been a week no breakup should I tell her even though I don't know her or just let him figure things out? Or should I wait for him to hit me up again to cheat to then go to her?
Should I tell her he cheated?
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