I have absolutely no idea. But one thing I believe I would consider is telling her (if you believe this) that you think you she should let her mother know, but the decision to tell her is absolutely hers and hers alone. You can think of it as cover, if you wish. But, if you don't tell your wife and nothing is ever said to her, it seems to me that you'd be in better standing with your wife if you told her you had left the decision to tell her up to your daughter. I agree with everything said above and would only add that if you tell her that you'll leave the decision to tell your wife up to her, that she is going to by-God talk to either you or her mother about it. And then, I'd take the suggestions in this thread about telling her in a non-threatening way that she's lucky to have learned this lesson without suffering any permanent damage.

Please keep in mind the fact that I am totally unqualified to offer advice when you are reading the above.