if shes got incriminating texts, shes showing other signs of guilt for whatever it is between them that he will pick up on.

You shouldn't involve yourself, but if he absolutely requires evidence that he hasn't gotten shadily in the first place, then just tell him to watch for some other sign of guilt, but he should act now and get it over with, the hell with how he figured it out.

Nothing worse than screwing around in a relationship that you know you are ending anyway, which it sounds like this guy is doing, better to move on without all the nonsense.

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