What would you do?

If you dated a guy and found out that he was seeing another girl (started dating both girls at the same time) would you text the girl to let her know? Or decide it is not your problem what she does and let them be?
My friend is struggling with a problem like that and as a third and more objective party I would probably let the other girl know (Not as revenge because if she didn't know either there would be no reason for an argument, but because i would feel bad for her). But i'm not very experienced in these things and I don't really know if letting her know would be the best option. I mean if i were her i would want someone to tell me that my boyfriend is cheating. Have you had any similar experiences? What did you do? And what was the result?
What would you do?
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