So, say it was before you were married. I tend toward waiting till she is ready to talk. She should also feel that you will be there solely to listen unless asked to do otherwise. We know that anything that effects her effects you. But it's probably going to effect her much more severely than you, so her needs must take priority. [Have I dealt with the effects? SSDD!]
However your tone drives me to think the other way that the relationship was during your marriage. This I didn't get to talk about with my first wife because I didn't understand until she was gone! If a sufferer is married and has an affair, they are fully responsible for your loss of trust. Unless you are codependent, they bear the full responsibility for regaining your trust. Suppose that you had an affair of your own... Then you are totally responsible for their loss of trust and for regaining it. The fact that they cheated first has no bearing on the issue. I don't think it maters how they cheated in most senses. The important thing is how are they going to regain your trust. Once your trust is regained, it might be helpful to investigate some of the conditions, but because your trust has been reestablished, I'd pursue it as if it happened before the marriage (see above). What ever triggered the colapse may need real attention in the future.
Bear