It would depend on the circumstances ie; If I was out at a restaruant and saw the offending party (for lack of a better term) with their cheatie, I would acknowledge them, walk up and say hello, talk to them, introducte myself to the cheatie(if I didn't know them) then leave it up to the offending party to spill the beans.
If ask by the offended party if I had knowledge, I'd tell them that they needed to talk to the offending party. If for some unforseen circumstances the offended could not talk to the offending, I'd then ask the offended are they sure they really want to know? Some things are best unknown, if the offended still prosisted then I would tell them and let the chips fall where they may. I'll not lie, cheat, or steal for anybody, not that I'm perfect but that is my goal no-matter how unattainable it is for mere mortals.
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"the memories of a man in his old age,
are deeds of a man in his prime"