Don't feel the need to tell someone their gf is cheating?

So a girl I know, wouldn't really call her a friend but an acquaintance, cheats away on some guy I don't know. Anyway I don't feel the need to tell him and I do not know why. If a gf of a friend of mine or someone I talked to was cheating I would feel the need to tell him but not when I only know of him. Any reasons why this is?

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