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

Lot of people have given really good advice. Firstly listen to what she has to say and make sure she realizes that you are there for her. In such situations it is very easy for someone's mind to run off track (revenge, suicidal thoughts etc). Help her remain positive throughout this ordeal. When such things happen it might get difficult to focus on work so cut her some slack. May be give her some leave if she wants.