Originally Posted by Daniel_

Of course, taking evilmatt's idea a step further, call the friend, tell them that your wife admitted the threesome, and tell them that you would love to invite them down to return the favour.

People who know there's a secret will group together to keep each others secrets. Once someone thinks the secret's out, they're relived to find that it's all over so they confess.

Failing that, also you can tell your wife that you've spoken to her friend who let it slip that she's had sex whilst up there, and you want to clear the air be her explaining exactly what she did in order to get through it.