How would you counsel your FDW if she tells you that her husband is seeing another woman?

It is impossible for us to know what exactly happened, the context and all that. I agree with what everyone suggested: to provide a listening ear. I may come off harsh by saying this, but you should not get too involved in domestic issues of others. Suggestions and advice will probably be good, but should you come in and things get worse, your FDW may just blame you for it.