This happened to me. I observed some extremely inappropriate public behavior from some XYZs. I don't know any alumnae personally, so I PM'd a couple of Greekchat XYZ alumnae so that they could decide whether any action was warranted. They were grateful for the heads up, and I believe they got in touch with the chapter's advisors and some changes were made.
I don't think it could hurt for you to tell your ABC alum friend what you heard (or, for that matter, to tell a Greekchat ABC alum). If she thinks the girls need some guidance, she can always get in touch with that chapter's advisors, or an ABC consultant, who will be more persuasive. At any rate, if you let someone know, then an actual member can decide what's best for ABC; right now they don't know what's going on, so they can't make that decision.
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