@wtf to do,
You seem awfully eager to be judge, jury, and executioner here, and based upon stuff that seems pretty damned flimsy.
Whatever happened to, you know, talking to your wife? Not about infidelity, more like -
are you happy?
As for not being able to say anything, why the **** not?
This is your marriage we are talking about here. If this so-called friend has sworn you to secrecy or some other such nonsense, check that person's agenda. They have put you into an untenable position and you need to tell them,
screw it, I am talking to my wife and you are not going to stop me.
Why? Because your primary loyalty should be with your spouse and not some pal who, like I said, seems to have an agenda. But you seem eager to either hire a PI to get your wife caught in
flagrante delicto or head straight to divorce court without such proof. Hey, if you wanna do that, by all means, go and knock yourself out. But if it turns out your wife is innocent and you're overreacting, or this friend was out for revenge on your wife and was all too happy to manipulate you, or anything of that nature, then don't cry if you've got Buyer's Remorse.
Divorces are no-fault in a lot of the United States. But it doesn't mean they are easy, painless, and/or free. Wanna pay spousal support, and child support if there are any kids? Wanna maintain two households? Then by all means, go ahead without any real proof, and see what happens.